Massive slips discovered in Hunua Ranges pose further threat to Auckland dams

Massive landslips and swathes of destroyed native bush have been discovered in Auckland's Hunua Ranges following the storms in early March, with Watercare warning they pose a further threat to the city's fragile water supply.

The scale of the destruction caused by the storms on March 7-8 has been revealed in drone footage and pictures taken by Watercare.

Imagery captured on Tuesday shows slips hundreds of metres long tore through commercial forests and native bush in the Hunuas in south east Auckland during the massive flooding event, which meteorologists have dubbed the "Tasman Tempest".

And prolonged rainfall expected over the next week means waterlogged soils are poised to dump further silt into Auckland's water supply.

Watercare chief executive Raveen Jaduram said the video footage was very concerning.

"With large areas of exposed soil and water-laden earth, further rainfall - as forecast by the MetService for this coming weekend - elevates the risk of more silt washing into our water supply dams."

Rain is likely over much of the North Island on the weekend, with the possibility of heavy falls on Sunday in Auckland and rain continuing through till Friday next week.

But the rainfall won't be in the same ballpark as the unprecedented March 7-8 downpour. Watercare data suggests more water fell in the first 24 hours of the Tasman Tempest than in the first day of Cyclone Bola in 1998. Between March 7-12 the Hunuas received more than two and half times as much rain as they did during Cyclone Bola.

Watercare and Auckland Council were looking for ways to manage the slips and protect the dams, Jaduram said. He is asking Aucklanders to keep saving water in light of issues at the Ardmore treatment plant, which normally supplies up to two thirds of the city's water supply but is still running well below treatment capacity due to excess silt in the storage dams.

In the days following the downpour, the water supply situation was reported as fragile - with the Ardmore plant treating half its normal volume of water and the other four metropolitan plants running at maximum.

Watercare called for Aucklanders to cut their water use by 20 litres per day, to avoid the company being forced to supply partially treated water, which would need boiling before use.

By March 15, water supply was stable. The volume treated at Ardmore has climbed by 30 million litres per day over the past week, but silt levels remain extremely high, Jaduram said. With more rain on the way, the plant's output will not be increasing further this week.

That meant people still needed save 20 litres a day, he said. In the past seven days Auckland used an average 404 million litres per day - slightly over the target of 400 million litres but well under the previous March average of 450 million litres.

Businesses normally ramp up their water use midweek, but on Tuesday - which was World Water Day - usage was still only at 406 million litres. By comparison, on March 7, the city used 424 million litres.

Even Auckland's youngest residents are getting in on the act. Kids at Lollipops Educare in St Lukes celebrated World Water Day by colouring with pens instead of watercolours and using leftover drinking water on plants.

Jaduram said it was "heart-warming" to see Aucklanders' response.

"We're a big city and yet we're proving that in times of need, we can come together and do our bit to preserve the health and wellbeing of our diverse community."

Auckland's wet week

Saturday: Rain developing afternoon.
Sunday: Rain at times, chance heavy.
Monday: Rain with northeasterlies.
Tuesday: Showers with northeasterlies.
Wednesday: Showers. Northeasterlies.
Thursday: Rain with easterlies.
Friday: Showers. Northeasterlies.