If Aucklanders fail to reduce daily water usage due to issues caused by this week's rainfall, a boil water notice will be needed in the city, warns Watercare.

Photo: Supplied / Auckland Council

More than 220mm of rain - or two months' worth - fell in 12 hours from late Tuesday in the Hunua Ranges, which feed the Ardmore Water Treatment Plant, causing a large amount of sediment to go into one of the lakes.

The city's water company has asked people to reduce their daily water usage by 20 litres a day, including by having shorter showers and turning off taps while brushing their teeth.

It was the first time there had been water restrictions in New Zealand's biggest city since 1994, the company said.

Staff were working 24 hours a day to manage the issue - and while some savings had been made, more were needed, it said.

Aucklanders use about 450 million litres of water a day, and yesterday's consumption went down to 420 million litres. The target that has been set is 400 million litres.

Watercare said a sustained period of dry weather was required in order for the water in the dams to settle, so the Ardmore plant could resume full activity.

There is currently a severe rain warning in the district, with heavy rain expected until the end of the weekend.

Watercare spokesperson Mark Bourne said it should take about a month for the water to clear after the "extraordinarily rare" rainfall.

At a news conference this afternoon, he said it was still safe to drink water straight from the tap and it was the quantity, not the quality, of the supply that had been compromised.

"Our target is 400 million litres [a day]... Savings must be ramped up and continued."

'Unique' situation

Mr Bourne said the "very worst case scenario" would be a boil water notice.

If savings were not met, partially treated water that did not meet drinking standards would have to be released from Ardmore, he said.

"I stress 'partially treated water' - it will still be chlorinated, but it will not longer meet New Zealand's stringent drinking water standards."

If that happened, it was "likely" a boil water notice would be issued.

"This is really unique for Auckland. The last time there was a need for any sort of water savings in Auckland was ... over 20 years ago, in 1994."

Mr Bourne said the water savings in the first 24 hours had been a good sign and, if Aucklanders stuck to the amount of water they used in the winter months, the target would be met.

"We all have to play our part," he said.

Photo: RNZ / Edward Gay

Twenty litres of water is equivalent to two buckets' worth.

Auckland mayor Phil Goff said if tonight's rainfall put more pressure on the city's largest water treatment plant, the way water was managed at Ardmore would need to be reassessed.

He said this week's storm might be called a one-in-100 year event, but if it was to become more common as a result of climate change, then new measures would need to be taken.

Advice for reducing water usage

Cut your shower time by two minutes: Approximate saving = 16 litres

Turn off the tap when brushing your teeth. Approximate saving = 4 litres

Don't use your toilet to flush rubbish. Approximate saving = 7 litres

Only run your dishwasher when it's full

Only run your washing machine when it's full

Source: Watercare