Planning ahead for Christmas Day with indoor or outdoor dining can all depend on what the weather is doing.

Fronts and an associated low crossing over the southern South Island are expected to bring rain or showers to many parts of New Zealand on Sunday, with possible thunderstorms over the North Island, says a MetService spokesperson.

There is moderate confidence that rainfall amounts about Westland could reach warning criteria - mainly about the ranges - on Sunday, and low confidence for northern Fiordland, Buller, western Tasman/Nelson, the Richmond ranges and Marlborough Sounds.

A Severe Weather Watch has already been issued for heavy rain about the Westland ranges on Sunday and early Monday.

Thunderstorms may also produce localised downpours over parts of the North Island on Sunday afternoon and evening.

Christmas Day, Monday, December 25

MetService is forecasting that on Monday December 25 an unsettled and humid northwest flow is expected to cover northern and central New Zealand.

"Rain or showers are expected in many areas, especially northern and western parts of both the North and South Island," says a MetService spokesperson.

"Thunderstorms are also possible in both islands."

MetService says there is low confidence that rainfall amounts could reach warning criteria about Westland and Buller, the Tararua Range, and the central North Island high country including Mt Taranaki and the ranges of eastern Bay of Plenty.

"Thunderstorms may also produce localised downpours over the northern North Island, mainly in the afternoon and evening."

Tuesday, December 26
Unsettled conditions are likely to continue on Boxing Day - Tuesday, December 26, with northwesterlies over the North Island and southerlies developing over the South Island.

"Rain or showers are expected in many areas. Thunderstorms are again possible in both islands."

There is low confidence that rainfall amounts will reach warning criteria about the central North Island high country, including Mt Taranaki and the ranges of eastern Bay of Plenty.

Wednesday, December 27
A southerly flow should cover the North Island on Wednesday, while a ridge spreads onto the South Island.

"At this stage, there is minimal risk of severe weather."