Auckland's water savings stepped up yesterday, with 391 million litres passing through the taps, under the target range of 400 million litres of less.

People are being asked to save 20 litres a person per day to avoid a boil water order while the company responds to elevated silt levels in its Hunua dams and at the Ardmore water treatment plant.

Aucklanders are continuing to do a phenomenal job of reducing their water use in the wake of the 'Tasman Tempest' weather event, Watercare said in a statement.

The council-owned water company has been able to increase the output of the Ardmore water treatment plant by 10 million litres a day.

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Even so, the savings call is expected to last two to three weeks.