Inside Ardmore plant, where Auckland's dirty water is treated

Auckland's Ardmore water treatment plant is processing the dirtiest raw water seen in its history, two weeks after heavy rainfall brought excess silt into city dams.

For days on ends, Aucklanders have been hearing one message - conserve water, or risk having to boil it.

Following all the rain a couple of weeks back, there's a very good reason.

"We've never seen water like this before. So, this is the first time that we've had siltation or silt build-up - particles in the water - to this level," Watercare operations manager Priyan Perera told Newshub.

The Ardmore plant is 60 years old, meaning this month's event even surpasses Cyclones Wilma and Bola.

The workers are used to filthy water, but extra staff have been called in to ensure they can keep turning the water into something drinkable.

"We've been manning the plant 24/7," Mr Perera said.

"There's been a lot of work taking place in the lab here where guys have been doing a lot of testing to make sure they can optimise the plant, given the amount and the change of the water we're seeing here today."

Ardmore is one of four treatment plants used to process Auckland's water.

Water flows in from the Hunua dams and debris is removed in Ardmore, where it's clarified and filtered, then disinfected.

A huge amount of sludge is removed from the water during the treatment and because of all the silt still flowing into the dams and through the treatment plant, they're having to clean it all out every couple of days rather than every week.

Since the floods the Ardmore plant has been working harder to produce the usual amount of clean water, so Watercare has the option of easing the load by drawing upon other dams and plants.

Watercare says ratepayers won't be affected by the extra workload because water charges are fixed, but the organisation will have to absorb any extra costs.

"The operating envelope for the plant will change, given we've had this event, so we'll have to take that step back and determine what we'll do for the future - what we have to do for Ardmore - given that that envelope has changed," Mr Perera said.

It's a matter of when, not if, the next monumental weather event strikes, and Watercare says it's already started planning for it.