Half-full dam now overflowing after downpour. Video / Alex Burton
The downpour flooding West Auckland homes overnight was so heavy that one of the city's drinking water dams has gone from being half full to overflowing in just 12 hours.
"Upper Huia Dam was just 45 per cent full at 9pm last night, and by 6am it was spilling," Watercarechief executive Jon Lamonte said of the reservoir in the Waitakere Ranges in Auckland's west.
"We've had more than a month's rain in 12 hours in our higher Waitākere catchments."
The downpour soaked western Auckland town Kumeū, which had its second wettest day since records began as stormwater flooded homes and cut off roads. Kumeū, Huapai, Rānui, Piha and Henderson Valley appeared the worst hit.
Weather forecaster Niwa said the area copped 201mm in just 14 hours - and a staggering 149 per cent of the normal August monthly rainfall in a single day.
Firefighters even resorted to using jet skis to reach trapped motorists on the Twin Coast Highway.
But although the rain has been bad news for many people, properties and stranded animals, it is good news for Auckland's water supply.
The catchments feeding into Waitākere, Upper Nihotupu and Upper Huia dams all recorded more than 200mm of rain in just 12 hours and they are now spilling over.
Auckland is still recovering from drought, but the heavy rains provided a much-needed boost to water supplies.
"Total dam storage was at 72.1 per cent at 10am this morning, up from just 64.9 per cent before the downpours began — an increase of more than 7 percentage points," Auckland mayor Phil Goff said.
Watercare's Lamonte said the city's western dams are much smaller than the Hūnua dams in the east, and therefore fill up much more quickly in heavy rain.
By way of scale, Watercare explained that all five of its western Waitakere dams would fit into the biggest of its eastern Hunua dams - twice over.
The Hūnua catchments also received between 30mm and 50mm overnight.
"While this rain is certainly helpful in terms of our water storage, we are still recovering from drought and we need to continue to be mindful of our water use," Lamonte said.
"Aucklanders have done an outstanding job over the past year, and we'll need to keep up the great work with these water savings, especially when the days start getting longer and warmer."
Meanwhile, Goff said Auckland Emergency Management and emergency services team are working to support those affected by flooding.
Woodhill farm stock on a little bit of the high ground left from flooding. Photo / Brett Phibbs
He asked everyone to stay safe and call 111 if they are in immediate danger.
"The flooding is also affecting Watercare's ability to deal with wastewater overflows. These can be reported to Watercare as normal and will be addressed as soon as it is safe to do so," he said.
Lamonte said the deluge has caused varying water quality at Waitākere Dam, and as a result, the Waitākere Water Treatment Plant will be shut down for at least the next 24 hours.
The rain also means Auckland's wastewater network has been inundated with stormwater, which is causing wastewater overflows in West Auckland.
Flooding at Kumeu in West Auckland. Photo / Brett Phibbs
"Our crews can't deal with these overflows until the flooding has subsided, but please report any overflows on our website – we will clean these up as soon as it is safe to do so," he said.
"This may take longer than normal, particularly with more rain expected and our backlog growing."
