Motorists were being urged to take care on the way home tonight - but the worst heavy rain is still to come tomorrow.

Severe thunderstorm warnings are in place for the upper North Island north of a line from Hamilton to Tauranga, and a heavy rainfall warning applies to the entire North Island and the north and west of the South Island.

The heaviest rainfall so far has been 70 to 85 mm in Taranaki and the Bay of Plenty, with some surface flooding also reported around Wellsford and Warkworth north of Auckland.

An NZ Transport Agency spokeswoman said no issues with surface flooding or weather-related incidents had been reported anywhere on the Auckland motorway network so far.

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"But in anticipation of bad weather, we advise motorists to slow down, take extra care, observe safe following distances and turn their headlights on," she said.

However there was flooding on State Highway 1 at Dome Valley, north of Auckland.

Other incidents around Auckland were adding to congestion.

A breakdown in the Victoria Park Tunnel around 7pm was blocking the righthand northbound lane, slowing traffic down.

The Southern Motorway was slow heading south between Highbrook and Greenlane, with police attending to a breakdown just before the Highbrook onramp.

An earlier crash on the Southwestern Motorway near Queenstown Rd has now been cleared but traffic is still heavy heading north.

MetService meteorologist Georgina Griffiths said the heaviest rain across the North Island, including Auckland, was expected between about noon and 9pm tomorrow.

"We are only just at the start of this event," she said.

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"There is some heavy rain coming in right now. We have a heavy rain warning in place and the numbers are relatively large for Auckland, so we'd expect 100 to 150 mm from 6pm tonight to 9pm tomorrow night in a lot of the region.

"Also the region is very saturated. After the extremely wet March, everything is super-soggy.

"There is a risk of localised downpours right across the region, and because the region is so very saturated we expect localised flooding, potentially flash flooding."