A warm, humid northeasterly flow covers New Zealand today, with a deep low over the Tasman Sea slowly approaching the country.
Brisk winds have started to sweep over the top of the country and rain was poised to hit western regions of both islands from this morning.
A MetService spokeswoman said there was a moderate risk of isolated thunderstorms developing in Southland and Otago later this afternoon and into the evening.
If the thunderstorms eventuated, they would be accompanied by brief heavy rain of 10mm to 20mm per hour and small hail.
Western and northern districts of both islands were bracing for a deluge in coming days, when up to 240mm of rain is expected in parts of the South Island and 150mm at the top of the country.
Motorists driving on rain-slicked roads in areas that haven't seen rain for weeks are being told to watch for "summer ice" as dust and oil build-up combined with rain cause tyres to lose traction on the greasy surface.
Heavy rain warnings have been issued for Northland, Nelson and Westland as the front moves slowly across the country today and tomorrow. Watches are in place for northern Westland, Buller, northern Marlborough and Mt Taranaki.
The heaviest rain is expected in the Westland ranges south of Otira, and Nelson from Motueka westwards. Up 240mm will fall in the coming 24 hours.
It's also likely to get extremely heavy at the top of the country, where up to 150mm is expected to drench Kaikohe north from this evening.
Other western regions will also get rain. Much of the North Island will get drenched as the wet weather moves east tomorrow and Friday.
MetService said although it was expected to be a wet week for many, eastern regions in the South Island remained sheltered by the Southern Alps.
The country would also be buffeted by strong northerly winds, particularly about coastal areas.
Ferries crossing Auckland's Hauraki Gulf have been cancelled until further notice as conditions are too stormy for safe travel.
- additional reporting by John Lewis
