Revellers should make the most of a mostly dry New Year's Eve, because the weather is set to "go downhill'' tomorrow, forecasters say.

MetService meteorologist Daniel Corbett said things were looking fine in most places until about midnight.

"The brief fine and dry spell across much of the country during the day on Tuesday and into the midnight hour will be short-lived as the next active trough spreads more rain and thunder across the country during the day on New Year's Day.''

There was a moderate risk of thunderstorms in Auckland tomorrow afternoon and small tornadoes were possible.


Strong to severe thunderstorms could also hit many western parts of the country during the day, with downpours possible in Northland.

"The weather system will have some grunt to it which will help generate some heavy rain and thunderstorms, especially in western parts of the country during the end of the day,'' Mr Corbett said.

Eastern parts of the North Island would see the country's best weather coming into the new year.

MetService is set to issue severe weather warnings for heavy rain in the Buller and Westland ranges and a severe weather watch for parts of the Nelson region tomorrow.

However, WeatherWatch head analyst Philip Duncan said tonight and tomorrow were looking "really good'' for Auckland.

"Mostly sunny weather, dry, with a 20 per cent chance of an isolated shower.''