Motorists are being warned of black ice across the North Island after the mercury plunged below zero in many places and Auckland hit near-zero temperatures for the second night running.

Temperatures across the country dipped near or under zero with Auckland's overnight low of 1.3C managing to nudge out Saturday night's chilly 1.5C.

MetService forecaster Tuporo Marsters said at least two Auckland suburbs - Albany and Takanini - hit zero just before daybreak.

Last night was the coldest night since July 2015.

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The bone-chilling wintry conditions were felt throughout the North Island with many inland areas covered in frost and recording temperatures well below zero.

Paeroa was -2.5C and Rotorua an even chillier -3C.

Marsters said Mt Cook recorded the coldest recorded temperature with -8C. The village barely managed to reach zero yesterday, he said.

The sub-zero icy conditions were also posing problems for motorists throughout the North Island, with roads turning treacherous from black ice.

The NZ Transport Agency this morning warned motorists to be wary of slippery patches of road across the Coromandel Peninsula.

Black ice was also starting to cause problems on Bay of Plenty roads with cars spinning out on slippery surfaces.

A police spokeswoman said a vehicle had slid on ice on State Highway 30 south of Rotorua and had ended up flipping on to its side around 6.30am.

A second crash north of the city car an hour later left a car also flipping on SH36 in the Ngawaro area.

No one was injured in either accident, she said.

But this was probably the last night of the bitterly cold temperatures for the north with a warm northerly wind bumping up temperatures across the country a degree or two today.

A heavy rain warning has been issued for Fiordland and Westland with the prospect of more snow for the south over the central South Island tomorrow.

Rain was already proving hazardous with the Milford Rd closed due to an increased avalanche risk.

This morning MetService extended the rain warning to include the ranges north of Wellington with up to 100mm of rain expected to start falling for 16 straight hours after midnight.

By midweek Northland, Auckland and the Coromandel Peninsula were set to be deluged with MetService expecting rain warnings to be issued.

Moderate rain is expected on Tuesday across the Kapiti Coast and southern parts of Manawatu.

"Given the sodden conditions, even lesser amounts such as 25mm to 40mm falling in under 12 hours could cause further issues, such as surface flooding and slips," MetService said in a weather watch.

"People in these areas are advised to stay up to date with the latest forecasts in case this watch is upgraded to a warning or further areas are added."

Today's forecast:

Auckland:

Morning cloud, fine with light winds. 13C

Hamilton:

Morning cloud, fine with light winds and early frosts or fog. 12C

Tauranga:

Fine with morning frosts, light winds. 13C

Wellington:

Cloud increasing, a few late showers. Gusty northerlies. 12C

Christchurch:

Fine with frosts, then high cloud increasing. Evening showers and northerly winds. 14C

Dunedin:

Cloud increasing with afternoon showers, then becoming fine. Fresh northerlies turning westerly. 12C