The frosty nights are over and there's unsettled weather on the way including heavy rain, strong winds, thunderstorms and even - maybe - tornadoes in the north.

A ridge that has been sitting over New Zealand brought clear skies and plummeting temperatures yesterday, with Upper Hutt dipping to -4.9C on Monday morning - its coldest August temperature in 53 years, according to Niwa.

But while northerly winds are bringing warmer temperatures forecasters say the weather is set to deteriorate with strong wind and heavy rain predicted.

MetService said there was also a possibility of thunderstorms late this afternoon and evening from Auckland north, with small tornadoes possible in Northland tonight. Strong winds were also likely from Auckland north.

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A western front followed by a low-pressure system meant everywhere would get some wind and rain this week, MetService Meteorologist Andrew James said.

The heaviest falls would probably be on the west coast of the South Island, where heavy rain watches and warnings were in place.

Central and southern parts of the North Island were also in line for heavy rain, James said.

Strong northwest winds were also expected especially near the Southern Alps today.

The combination of heavy rain and strong winds at high tide, along with low air pressure could cause some coastal flooding tonight for low-lying areas of Nelson, Buller, and Westland, according to MetService.

A slip in Nelson after torrential rain in December 2011. Parts of Nelson have a heavy rain watch, with MetService saying rain may reach warning levels tonight and coastlines may flood. Photo / File
A slip in Nelson after torrential rain in December 2011. Parts of Nelson have a heavy rain watch, with MetService saying rain may reach warning levels tonight and coastlines may flood. Photo / File

Heavy rain in Westland and Fiordland could also see streams and rivers rising rapidly, with surface flooding and slips possible.

Nelson, the Buller and Westland ranges, the Otago headwaters and the Canterbury headwaters north of Arthur's pass were also under heavy rain watches throughout today and early morning Wednesday, according to MetService.

Strong winds in Canterbury's high country and Fiordland could damage trees and powerlines, the forecaster warned.

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Southern Lakes, Central Otago west of Alexandra, and Southland also had strong wind watches in place.

Weatherwatch said the low would get deeper over Wednesday and Thursday, making for unsettled weather and bringing the possibility of thunderstorms, squalls, heavy rain and snow.

By Friday that low would fall apart and by Saturday conditions would begin to improve for most of New Zealand.

Today's weather

Whangārei

Cloudy periods, chance shower. Rain developing afternoon. Northeast strengthening then changing westerly at night. High 15C, Low 10C

Auckland
A few showers. Rain developing evening. Northeast strengthening then changing westerly at night. High 15C, Low 10C

Hamilton
Cloudy. Rain developing at night. Northeast changing westerly at night. High 13C, Low 8C

Tauranga
Cloudy, morning drizzle. Rain developing at night. Northeasterlies. High 14C, Low 10C

New Plymouth
Mostly cloudy. A few evening showers then rain at night. Northerlies strengthening, then changing westerly at night. High 13C, Low 8C

Napier
Some morning cloud then fine. Northeasterlies. High 15C, 8C

Whanganui
Mostly cloudy, then rain at night. Northerly breezes. High 16C, Low 8C

Wellington
Mostly cloudy. Rain developing at night. Northerlies, strong at times. High 12C, Low 8C

Nelson
Showers, turning to rain this evening. Northerlies gradually strengthening. High 12C, Low 7C

Christchurch
High cloud increasing with strong northeasterlies. A few spots of rain developing and northerlies easing at night. High 15C, Low 4C

Dunedin
Cloud thickening. A period of afternoon rain, then fine spells. Strong northeasterlies, changing lighter northwest afternoon. High 11C, Low 3C