A long hot summer could suck Wellington dry as unexpected population growth strains the region's water supply.

Greater Wellington regional council's long-term plan says the council must be able to withstand a one-in-50-year drought. But it failed to hit that target in the past financial year, managing only a one-in-26-year standard, prompting alerts to residents to start conserving water now to avoid restrictions later.

Marketing analysis manager Andrew Samuel said the standard was worked out from projected population growth and water usage against climate records, but in recent years, Wellington's population growth had continually exceeded projected levels.

Growth modelling done by the council in 2000 indicated the population of the region's four cities would not reach 377,000 till 2020.

By June this year the estimated population was 389,700.

Because the region's water came from rivers and an aquifer, supplies would be severely stretched if there was a lack of rain, Mr Samuel said. "It really starts to put extra pressure on the system." That would mean cracking down on water use.

For most of the year the region used about 150 million litres of water a day, but that leapt to 200 million litres during peak days in summer.

The easiest way to avoid restrictions was to start conserving water now, he said. "It's the difference between a soft landing and a hard landing."

The council had started a programme of identifying and fixing leaks, but chairwoman Fran Wilde said it was up to residents to take responsibility.

"People can actually use less water voluntarily ... It saves the ratepayers having to build a big dam straight away."

Wellington City Council infrastructure manager Maria Archer said getting into good habits would delay the need for water meters. "If nothing changes, we'll have to start thinking about it quite seriously in four or five years."

Get Conserving:

Target watering – only water gardens in the morning and evenings, and aim the water at the roots.

Be a sensible flusher – where appropriate, use the half-flush option. If your toilet does not have one, fill a litre bottle with sand, and place it in the cistern to reduce the amount of water used in each flush.

Nozzle your hose – this stops unnecessary spraying even if you leave the tap on. A hose uses 150 litres every five minutes.

Quicker cleans – showers and clothes washing are the biggest water wasters.

Take shorter showers, and wash only full loads of clothes.

The same goes for dishwashers. Stop leaks – check for leaks and fix them promptly. You could potentially save up to 5000 litres a month.

Turn the tap off – when brushing your teeth you use five litres.