Residents on Auckland's North Shore clean up after a powerful storm hit and flooded their home.

Two Auckland beaches have been contaminated with sewage following a storm which caused flash floods on the North Shore.

Swimmers were being urged to avoid both Milford and Castor Bay beach after heavy rain flooded the wastewater system, triggering an overflow, on Tuesday.

Auckland Council's SafeSwim website posted red alerts at the two locations, triggered by the monitoring alarm on the Watercare network.

SCREENSHOT Heavy rain has triggered sewage overflows at Milford and Castor Bay.

Watercare confirmed a wet-weather overflow occurred into the local waterway, which flows out to sea, wastewater operations controller Sifa Pole said.

READ MORE: Thunderstorms in Auckland, hot in south

"The deluge of stormwater engulfed the wastewater system.

AMY BAKER/STUFF Pyper Nielsen, 15, Nate Skinner, 14, and Phoebe Nielsen, 17, of Sucklings Lane in Albany kayak down the street during the flash flooding.

"These engineered overflows are designed to prevent wastewater from backing up and overflowing onto private property."

Are you affected? It it's safe, send us your pictures, videos and other information to newstips@stuff.co.nz

The morning's isolated rainstorm, which affected a large area of the Albany basin, caused alarmingly fast flooding, locals said.

LAWRENCE SMITH/STUFF Several cars in the area had been displaced by the flash flooding.

Sitting in a kayak on her flooded street, Albany girl Pyper Nielsen, 15, said: "It happened within five minutes. It was really quick."

Phoebe Nielsen, 17, said her first car was completely ruined by water, which rushed over driveways.

The water came extremely close to getting into houses on the street, she said.

Stuff Major flash flooding has hit Auckland.

Appar Singh was driving his girlfriend to Albany to work through Caribbean Drive and got trapped when his car got stuck in floodwaters just before 9am.

"I was like, 'I think I'll be able to take it out', and I just tried, and my car stopped."

He said his car started floating.

ZAR LILLEY/STUFF Storm clouds hang over Auckland on Tuesday afternoon.

The Glenfield resident didn't have insurance and said he didn't know what to do now.

Fire and Emergency NZ communications shift manager Scott Osmond said two people stuck in their cars in flood waters at the end of Caribbean Dr in Unsworth Heights had been freed. The road had since re-opened.

A car was stuck in flooding on intersection of Upper Harbour Highway and Caribbean Dr, while another was stuck in flooding on Rosedale Rd.

LAWRENCE SMITH/STUFF Residents on Auckland's North Shore clean up after a powerful storm hit and flooded their home.

Two houses also experienced flooding due to overloaded drainage. The Fire Service was on the way to a third.

Rosedale Rd, where another car was reported stuck in flooding, had 52mm of rain fall in an hour since 9am.

Oteha Valley Rd was impassable after flooding.

TAMARA THOMPSON Floodwaters cover Caribbean Drive.

Scott Osmond said flooding occurred all around the Albany basin.

Firefighters had about a dozen callouts, mainly flooded roads, people trapped in cars on Upper Harbour Drive and Rosebank Rd, and houses flooded in Clemows and Sucklings lanes in Rosedale, he said.

In most cases flooding was cleared by unblocking a drain or a spouting and water receded quite quickly.

Amy Baker / Stuff Sucklings Lane, Albany residents woke up to hyper-local flash flooding.

Osmond's rain radar showed rainfall lessening and heading in a northeast direction.

"Fortunately it seems to be easing off now," he said.

Earlier, there was a six-car crash on the Hibiscus Coast Highway about 7.30am after a heavy downpour.

MetService had warned of severe thunderstorms and heavy rain in Auckland.

The forecaster said thunderstorms detected by its weather radar shortly after 9am were moving west northwest and expected to be accompanied by torrential downpours.

It said the storms could produce localised downpours of 25 to 50mm per hour or more and there was a risk thunderstorms could form through to about 8pm on Tuesday night.

TAMARA THOMPSON Cars in the floodwaters of Caribbean Drive, in Auckland's Unworth Heights.

MetService meteorologist John Law said the unstable air, combined with warm and humid weather over the last few days had made the perfect conditions for Tuesday morning's thunderstorms – and the lack of winds meant they stayed in one place.

"A very slow-moving thunderstorm [has] sat over one spot and brought as much as 50mm in less than an hour.

"That brought a very heavy, very intense, very slow-moving burst of rainfall to the area."

Karey / Facebook The corner of William Pickering Drive and John Glenn Ave in Auckland's Rosedale area were completely awash during flash flooding.

Supplied Flash flooding has hit Auckland's Upper Harbour region.