El Nino and Water Tanks in New Zealand: What You Need to Know

If your home, farm or business relies on tank water, El Nino is worth paying attention to.

El Nino conditions are now established in the Pacific and are expected to influence New Zealand’s weather through spring and summer 2026/27. Earth Sciences New Zealand says the current El Nino is strengthening considerably and could become one of the strongest in recent history.

That does not mean every part of New Zealand is heading for a drought. As any Kiwi knows, our weather doesn’t always follow the script, and El Nino affects different parts of the country differently. However, below-normal rainfall, more frequent dry spells and reduced soil moisture and river flows are expected in several regions.

For anyone relying on rainwater tanks, that makes water management particularly important.

The good news is there is plenty you can do now. Understanding your water supply, making the most of the rain that does fall, and getting your tank and filtration system ready can put you in a much better position if things start to dry out.

What Is El Nino?
How Can El Nino Affect Rainwater Tanks?
Which Parts of New Zealand Could Be Most Affected?
How Much Rainwater Can Your Roof Collect?
Make Sure You Are Capturing as Much Rain as Possible
Know How Much Water You Have
How Can You Make Tank Water Last Longer During El Nino?
Should You Get a Bigger Water Tank?
Get Your Water Tank Ready Before Dry Weather Arrives
Can You Clean a Water Tank Without Emptying It?
Don’t Forget Your Water Filtration System
What If Your Water Tank Is Already Getting Low?
What About Bore and Spring Water?
El Nino and Farm Water Tanks
Prepare Your Water Tank for El Nino
Frequently Asked Questions

What Is El Nino?

El Nino is a naturally occurring climate pattern that develops in the tropical Pacific Ocean and influences weather around the world.

During El Nino, changes in ocean temperatures and atmospheric circulation alter prevailing winds and rainfall patterns.

For New Zealand, El Nino commonly brings stronger and more frequent winds from the west and southwest. However, the effects vary considerably depending on where you live.

According to Earth Sciences New Zealand’s September to November 2026 Seasonal Climate Outlook, rainfall is most likely to be below normal in the north and east of the North Island. Several other regions have an increased chance of normal or below-normal rainfall, while the west of the South Island is expected to be wetter than normal.

This is an important distinction. El Nino does not simply mean all of New Zealand becomes dry.

For tank owners, what really matters is what happens to the rainfall at your place and whether your water supply is prepared for longer gaps between decent downpours.

How Can El Nino Affect Rainwater Tanks?

If your tank collects rainwater from your roof, the relationship is fairly simple: your water supply depends on rain.

Longer periods without significant rainfall mean less water entering your tank while your household continues using water every day.

A tank that seems to happily keep itself topped up during wetter months can start dropping surprisingly quickly once the rain stops.

The amount of pressure this places on your water supply depends on several factors:

How much rainfall your property receives
The size of your roof catchment
How efficiently rainwater reaches your tank
The size and number of your water tanks
How much water your household uses
Whether your tank or plumbing system has any leaks
How much water you have stored when a dry period begins

This is why preparing for dry weather involves more than simply having a water tank. You want the whole collection and storage system doing its job.

Which Parts of New Zealand Could Be Most Affected?

El Nino does not affect every region in the same way.

The latest outlook from Earth Sciences New Zealand expects below-normal rainfall to be most likely across the north and east of the North Island during spring 2026.

Rainfall in the north and east of the South Island and west of the North Island is about equally likely to be near normal or below normal. The west of the South Island has a very different outlook, with above-normal rainfall considered very likely.

Below-normal winter rainfall has also reduced groundwater recharge in a number of regions. Earth Sciences New Zealand expects some of these areas to experience reduced rainfall, more frequent dry spells and further drying of soils and vegetation during spring.

The message isn’t that everyone’s tank is about to run dry. It’s simply that some parts of New Zealand face an increased risk of extended dry periods, so getting organised before summer is a sensible move.

How Much Rainwater Can Your Roof Collect?

One of the most useful things a tank owner can understand is how much water their roof can potentially collect.

As a general principle, 1 millimetre of rain falling on 1 square metre of roof represents approximately 1 litre of water before allowing for losses.

That means a 200 square metre roof receiving 10 mm of rain has approximately 2,000 litres of rain falling onto it.

Suddenly that overnight shower doesn’t look quite so insignificant.

Of course, not all of that water necessarily reaches your tank. Some can be lost through first-flush systems, overflowing gutters, leaks, evaporation or inefficient collection.

During a dry period, making the most of the rainfall you do receive becomes increasingly important.

Make Sure You Are Capturing as Much Rain as Possible

Before worrying about increasing your storage capacity, check whether your existing system is collecting water effectively.

A surprising amount of potential tank water can disappear before it ever reaches the tank.

Check:

Gutters for leaves and other blockages
Downpipes for obstructions or leaks
Tank inlet screens and sieves
First-flush diverters
Flush caps
Pipe fittings and connections
Tank lids and access points
Overflow pipes
Overhanging vegetation around your roof, gutters and tank

It is also worth checking your roof and gutters after significant winds or storms. A handful of leaves in the wrong place can quickly block an otherwise well-maintained collection system.

When the rain finally arrives, you want as much of it as possible heading into the tank rather than over the side of the gutter.

Know How Much Water You Have

Anyone who has lived on tank water for a while usually develops a pretty good feel for their supply.

But guessing becomes harder when conditions are unusually dry.

Know the capacity of your tank or tanks and keep an eye on the amount of water remaining.

You should also have a rough understanding of how quickly your household normally uses water.

If your tank level starts falling considerably faster than expected, investigate why. Increased household use may be responsible, but an unexpectedly rapid drop can also indicate a leaking pipe, dripping tap, running toilet, irrigation problem or leak elsewhere in the system.

A small leak might not seem like much on a wet July day. After several weeks without decent rain, every litre starts to matter.

How Can You Make Tank Water Last Longer During El Nino?

If rainfall becomes less reliable, small changes in household water use can make a meaningful difference over several weeks.

Depending on your household, this could include:

Fixing leaking taps, pipes and toilets
Taking shorter showers
Running washing machines and dishwashers with full loads
Reducing unnecessary outdoor water use
Giving the car wash a miss during particularly dry periods
Checking irrigation systems for leaks
Using water-efficient appliances and fittings
Keeping an eye on tank levels rather than waiting until water is almost gone

Nobody wants to spend summer counting every drop that comes out of the tap.

The idea is to understand where your water goes and cut out the waste before you need to make more difficult changes.

Should You Get a Bigger Water Tank?

El Nino may prompt some tank owners to wonder whether they have enough storage.

There is no single tank size that is right for every property.

The appropriate amount of storage depends on your local rainfall, roof catchment area, household size, water consumption, available space and whether you have another source of water available.

A larger tank provides greater storage capacity, but installing another tank won’t solve problems elsewhere in the system. If your gutters, downpipes or collection system are inefficient, increasing your storage may provide less benefit than expected.

Before adding another tank, understand how much water you can realistically collect and how much your household normally consumes.

Get Your Water Tank Ready Before Dry Weather Arrives

Your tank itself should also be part of your preparation.

Over time, sediment, organic material and other debris can accumulate at the bottom of rainwater tanks. How quickly this happens depends on your roof, surrounding environment, collection system and maintenance.

Check that your tank is properly sealed, the lid is secure and there are no obvious entry points for animals, insects, debris or sunlight.

If you can safely inspect the tank, check whether there is significant sediment accumulation at the bottom.

And if the tank needs a clean, it’s much nicer dealing with it now than discovering a problem when summer has arrived and the water level is already heading south.

Can You Clean a Water Tank Without Emptying It?

Yes, and this becomes particularly useful when every litre of stored water matters.

Traditional water tank cleaning methods can involve draining or pumping out large amounts of water. For a household relying entirely on rainwater, the thought of throwing away thousands of litres of perfectly usable water heading into a potentially dry summer isn’t particularly appealing.

Safe H2O uses a cleaning process designed to remove sediment and contaminants from the tank while retaining the existing water.

This means you don’t need to throw away an entire tank of stored water simply because the tank needs cleaning.

It’s useful at any time of year, but particularly helpful when preparing for summer or an extended dry spell.

If your tank is due for a clean, getting it sorted before conditions become very dry means your water storage system is ready when you really need to rely on it.

Don’t Forget Your Water Filtration System

Your tank is only one part of your household water system.

If you use tank water for drinking, check that your filtration and treatment equipment is operating correctly.

Depending on your system, this may include checking:

Sediment filter cartridges
Carbon filters
UV systems
UV bulb replacement dates
Filter housings
Water pressure
Pumps and associated plumbing

Filters require regular replacement, and UV systems need ongoing servicing to perform as intended.

It’s much better to discover a tired filter, faulty UV system or unhappy pump while you still have plenty of water than halfway through a long, hot summer.

What If Your Water Tank Is Already Getting Low?

If your tank is dropping faster than expected, start by working out why.

Check your tank level, look for leaks and consider whether your household’s water use has recently increased.

Then look at your collection system. If rain has fallen but your tank level has barely changed, check gutters, downpipes, first-flush systems and tank inlets for blockages or leaks.

If there simply hasn’t been enough rainfall, start conserving water before the tank reaches a critically low level.

Depending on where you live, you may also be able to purchase drinking water for delivery. If you think this could become necessary, planning ahead is better than waiting until the tank is virtually empty, particularly if half the neighbourhood has had the same idea.

What About Bore and Spring Water?

Not every water tank relies on rainwater.

Many New Zealand properties store water supplied from bores, springs, streams or other sources.

El Nino can still be relevant to these properties. Earth Sciences New Zealand has reported reduced groundwater recharge following below-normal rainfall in several regions, with further pressure possible as dry conditions develop.

However, the issues associated with bore and spring water are different from those associated with roof-collected rainwater.

If you rely on bore, spring or surface water, understanding both the availability and quality of your source is important. Laboratory water testing can identify chemical and microbiological characteristics that may affect how the water should be treated.

El Nino and Farm Water Tanks

For farms and rural properties, water planning can become even more important.

Tanks may be supplying livestock, washdown systems, irrigation and the house, so daily water use can quickly add up.

Farm tank owners should know:

Their total storage capacity
Approximate daily water consumption
Which tanks supply which parts of the property
Whether troughs and water lines have leaks
How quickly tanks can be replenished
What alternative water sources are available

Regularly checking tanks, troughs, pipes and connections can identify water losses before they become serious.

A leaking trough isn’t just an annoying maintenance job when things get dry. It can be a substantial amount of your stored water disappearing into a paddock.

Prepare Your Water Tank for El Nino

Nobody can predict exactly how much rain will fall on your roof over the coming months.

El Nino also doesn’t mean that every part of New Zealand will experience drought. There will still be wet periods, storms and plenty of regional variation.

What we do know is that the current El Nino is strengthening and is expected to influence New Zealand’s weather through spring and summer.

For those of us who rely on tank water, that’s a good reminder to get things sorted before the Kiwi summer really gets underway.

Check your gutters and downpipes. Know how much water you have. Look for leaks. Make sure your filtration system is maintained. Check your tank for sediment and other problems. And when the rain does arrive, make the most of every drop.

If your tank needs cleaning, Safe H2O can clean your water tank while retaining the existing water, helping you prepare your system without unnecessarily disposing of the water you’ve already collected.

Contact us to discuss your water tank, filtration system or water quality. We’re always happy to have a chat and help you work out what your property needs.

Frequently Asked Questions

Does El Nino Mean Less Rain in New Zealand?

Not everywhere. El Nino changes New Zealand’s typical weather patterns, but its effects vary considerably by region. For spring 2026, below-normal rainfall is most likely in the north and east of the North Island, while the west of the South Island is expected to experience above-normal rainfall.

Will El Nino Cause My Water Tank to Run Dry?

Not necessarily. Whether a tank runs low depends on local rainfall, tank capacity, roof catchment, household consumption and how much water is already stored. However, extended dry periods can reduce how frequently rainwater tanks are replenished.

How Much Water Can I Collect From My Roof?

As a general principle, 1 mm of rain falling on 1 square metre of roof represents approximately 1 litre of water before allowing for collection losses. For example, 10 mm of rain falling on a 200 square metre roof represents approximately 2,000 litres of potential rainwater collection.

Should I Get a Bigger Water Tank Before El Nino?

Additional storage may be useful for some properties, but tank size should be considered alongside local rainfall, roof catchment area, household consumption and available space. It is also worth checking that your existing rainwater collection system is working efficiently before increasing storage.

Should I Clean My Water Tank Before Summer?

If your tank is due for cleaning or contains significant sediment, having it cleaned before an extended dry period can make sense. Safe H2O’s cleaning process allows the existing tank water to be retained rather than requiring the entire tank to be emptied.

How Can I Make My Tank Water Last Longer?

Start by checking for leaks and unnecessary water use. Monitor your tank level, keep gutters and downpipes clear so you capture available rainfall, and reduce non-essential water use when your stored supply begins to fall.