Christmas Day could be soggy in parts of the country, with a severe weather watch for Fiordland and Westland.
The MetService says up to 50mm of rain could fall in six hours in those areas.
The wet weather will then move up the South Island towards the evening.
Evening rain showers are also expected in the North.
Tomorrow's forecast for Christmas Day.— MetService (@MetService) December 24, 2017
The MetService forecast said a ridge of high pressure that has been sitting over the country for the past couple of days produced mostly dry, cloudy weather. A front from the across the Tasman is expected to bring some wet weather.
Northerly and easterly regions can expect a drier start to Christmas Day and a hot one before rain arrives later on.
MetService meteorologist Tui McInnes said the approaching front was typical, with the Southern Alps trapping the moisture and heaviest rain on the West Coast.
"While it's unfortunate that the timing is for Christmas Day, there will be many places thankful to finally see some rain.
"The sheltering from the Southern Alps does keep eastern areas drier, but as the front progresses, regions like Otago and Dunedin will experience a period of wet weather."
In the north, most places are forecast to be mainly fine, but a weak trough os forecast to bring some isolated showers, particularly for inland and western places.
"The trough has been sitting out to the west of the North Island and is getting pushed back inland during Christmas Day. This does mean some showery conditions for places. However, these will be more isolated in nature and will mainly affect just central and western areas, keeping other places drier."
A ridge of high pressure moves over the country next week but the New Year forecast is for northwest winds and a wet start to the year for some regions.