Police are warning southern motorists to take extra caution, as the south faces potentially heavy frost.

"Police urge motorists to take care when driving into shaded areas, bridge crossings and areas that could potentially harbour the infamous black ice," Senior Sergeant Craig Dinnissen said.

"Remember to leave a bit earlier, increase following distances, avoid erratic steering movements or heavy braking."

Various sections of road in the southern region have blue catseye devices which indicate icy conditions, but police warn these are not the only areas where roads may be dangerous.

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"Police advise these conditions can last through the day so ensure care is taken on your trip home at the end of the day also," Mr Dinnissen said.

The south has been blasted by rain and snow showers, causing road closures and multiple accidents in the past couple of days.

However, MetService meteorologist Tom Adams said there was an improving trend in the south.

"It got fairly cold overnight; down to -2.7C inland at Lumsden and -0.7C at Gore," he said.

"There's high cloud there now and it's relatively clear but there's rain spreading across the South Island today. The rain will clear by Friday, bringing warmer temperatures."

Meanwhile, the north is facing potential rain, squalls, strong winds, thunderstorms and coastal tornadoes.

For Auckland, there is a moderate risk of thunderstorms about midday, which are likely to be accompanied by heavy rain, hail and strong winds up to 110km/h.

There is also a risk of severe thunderstorms from the Waikato to the Kapiti Coast this morning and MetService has warned there could be small coastal tornadoes.

The rest of the North Island can expect showery conditions, especially up the west coast from Taranaki to Northland, WeatherWatch analyst Philip Duncan said.

"Some of these showers will become heavy with some thunder at times during the afternoon and into evening."

Mr Duncan said gusty northwesterlies would swing to the west in the afternoon and temperatures may fall in some places.

"The east coast sees sunny areas and some high cloud, with the chance of a spot of rain spreading from the west at times. Otherwise it will be mainly dry with nothing substantial for farmers this time around."

MAIN CENTRES OUTLOOK

Whangarei:

Periods of rain, possibly heavy. Strong southwest change. High 20C.

Auckland: Periods of rain, possibly heavy. Strong southwest change. High 18C.

Tauranga: Rain, possibly heavy, and strong northwest winds, easing in the evening. High 17C.

Hamilton: Rain with a chance of thunder. Rain eases in the evening with a southwest change. High 16C.

Wellington: Rain clearing to become fine in the afternoon. Gale northwest winds ease. High 15C.

Christchurch: High cloud. Northeast for a time. High 12C.

Dunedin: Increasing high cloud. Northerlies. High 12C.

Invercargill: Few showers from afternoon, some possibly heavy. Northerly turns westerly. High 10C.

- Source: MetService