Three days of heavy rain downpours have brought Auckland's dams back to 51 per cent capacity, but Aucklanders were still being encouraged to conserve water.

The brief reprieve comes as the city moves to an alert level system to manage its water supply crisis.

A significant lack of rain, including 78 days with less than 1mm of rain from January 20 to April 6, has resulted in a serious water shortage at Auckland's nine dams.

The region has recorded less than half of its normal rainfall since November last year.

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Auckland Mayor Phil Goff feared without enough rain in winter and spring, the city's water supply would plummet to 200 million litres of water a day and bring severe water restrictions.

But the weather bomb that hit Auckland last night bringing heavy rain and tornadoes has helped to fill some of the dams, according to Watercare Services.

"We are delighted to see significant rain has fallen overnight in our western catchment, and now total dam storage is at 51.3 per cent. They were last at this level on April 8," a Watercare spokeswoman said.

On Friday in the Hūnua Ranges 4.5mm of rain fell, but between midnight to 8am on Saturday up to 250mm was recorded there.

For the same time period on Friday 20.5mm fell in the Waitākere Ranges and up to 60mm on Saturday.

In total, the dams were 49.2 per cent full on Friday and 51.3 per cent on Saturday. The normal capacity level is 78.5 per cent full for this time of year.

The target for June 2020 is 405 million litres or less a day. Yesterday's consumption was 405 million litres and the seven-day rolling average by Friday was 402 million litres.

"Our southern dams, in the Hūnua Ranges, do however still need significantly more rainfall and we'd like to encourage Aucklanders to continue with their water conservation efforts," the spokeswoman said.

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Under the four-stage alert system, residents in level face a ban on using hoses and water blasters outdoor, four-minute showers and a 410 litres a day usage target.

At stage four the restrictions would be severe and include savings increases per person to 40 litres a day and at least a 30 per cent cut in water use by businesses.

Ways to save water include spending a minute less in the shower to save 12 litres of water, turning off the water when you brush your teeth to save four litres, fixing a leaking tap which saves 33 litres a day and using a half-flush in the toilet to save about six litres.

Auckland Council asked planned to use special fast-track legislation to double its water allocation from the Waikato River, which drew criticism from Waikato-Tainui and Waikato Regional Council.

The council would push for resource consent for another 200 million litres a day despite Waikato Regional Council chairman Russ Rimmington saying Auckland wanted 80 per cent of what's currently left to be allocated, leaving the remaining 20 per cent for 300-plus applicants.

Rimmington said Auckland needed to consider more sustainable ways of water harvesting and collection and retention including through collecting roof rainwater and enlarging the dams.

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