Heavy rain forecast for much of upper North Island and eastern South Island

Brace yourself for some wet and wild weather today - with thunderstorms and heavy rain forecast for the upper North Island and eastern South Island.

A deep low crawling in from the west is bending wet northeasterlies across the country. The slow-moving system will last for days.

The general sogginess follows a late battering of wintry weather this week where snow fell on alpine roads and Mount Cook village shivered in an overnight low of minus 2C.

Rain is forecast today from Northland to Otago, with only Southland, and in the afternoon Westland, expected to see much sun.

MetService forecaster Ravi Kandula said that a heavy-rain "watch" was in place from 5am to 5pm today for the Bay of Plenty and Rotorua. A watch is a step short of a warning.

Kandula said that in Bay of Plenty and Rotorua there was the potential for "warning amounts" of rain - 50mm in the 12-hour period - to accumulate.

Heavy-rain watches were also issued for Northland to 9am today; Auckland, Great Barrier Island and Coromandel Peninsula to 1am; eastern Marlborough to 9pm tomorrow; Canterbury plains and high country and North Otago to 6am tomorrow.

Aucklanders can expect periods of possibly heavy rain today and even a chance of thunderstorms. But the downpours should ease to showers by afternoon.

Thunderstorms are possible in many North Island areas and temperatures should mostly be in the high teens or early 20s, and up to 23C in Auckland.

The South Island's east coast joins in the dousing today but temperatures are predicted to be down in the mid-teens.

Invercargill looks likely to escape the rain, with a forecast of fine breaks and 19C today.

Kandula said the "very slow moving" weather system would control our weather until about Wednesday, when it would start to move further east.