The rain watch suggested there could be durations of intense localised rain, up to 40mm per hour, with isolated thunderstorms a possibility.
Looking forward to the weekend, northwesterlies over the South Island would spread over the lower North Island on Saturday.
"A front approaches Fiordland from the southwest late Saturday, then becomes slow-moving about Fiordland or southern Westland during Sunday and Monday and gradually weakens," MetService said.
The front was expected to bring periods of rain to Fiordland and southern Westland during the weekend, then rain would spread north to Buller on Monday, new years eve.
"There is moderate confidence of rainfall accumulations approaching warning amounts about northern parts of Fiordland and southern Westland on Sunday, and the confidence reduces to low on Monday."
For the rest of the country, temperatures in the high 20s were expected about eastern areas of the country over the weekend, especially the South Island.
"On Tuesday, a ridge should cover much of central and northern New Zealand with afternoon showers about the ranges, and northwesterlies may develop in the south of the South Island."
The extended forecast for Cardrona over the Rhythm and Alps festival, December 29 to January 1, showed a moderate chance of rain through the festival.
Your weather
Whangārei: Fine. Southwesterlies.
High 24C Low 13C
Auckland: Fine. Southwest breezes.
High 23C Low 15C
Tauranga: Cloudy. Showers, mainly from afternoon. Northeasterlies developing.
High 23C Low 17C
Whanganui: Fine. Southerly breezes.
High 24C Low 14C
Napier: Cloudy with showers morning, clearing to fine breaks afternoon. Northeasterlies.
High 22C Low 16C
Wellington: Fine. Southerly breezes developing.
High 22C Low 15C
Christchurch: Morning cloud then fine. Northeasterlies becoming strong after midday.
High 20C Low 11C
Dunedin: Fine apart from evening high cloud. Northeast strengthening.
High 21C Low 15C
Source: MetService
