The MetService last night sounded heavy rain warnings for the West Coast of the South Island and Buller-Nelson and said it expected bad weather to then wash over parts of the northern North Island.
Spokesman Andy Downs said the slow-moving system rolling on to the South Island "has all theingredients for good heavy rain".
Canterbury manager Hayward Osborn said the approaching system appeared to be a "major event".
Mr Downs warned of "exceptional rainfall" for Westland and down to Milford Sound overnight, with the heaviest falls likely in South Westland.
He forecast up to 350mm by midnight tonight in that area, with rain falling at rates of up to 30mm per hour.
Rain was already falling on the West Coast yesterday - the Westport trots were in rain on a sloppy track.
About 100mm is predicted in the catchment areas of Canterbury and Otago lakes and rivers.
Mr Osborn warned that river levels might rise quickly. The potential for flooding in Westland was increasing and snow melt could create similar problems on rivers flowing across Canterbury and Otago.
"It's important that trampers, campers, travellers, boaties, farmers - anyone considering outdoor activities over the next two days - keep in touch with the latest developments."
Mr Downs said the weather system coming in from the Tasman was moving slowly. "Normally the systems go through quickly but this one isn't and it's packing a lot of moisture."
That meant it had the capacity to dump a lot of rain over an extended period. The front, moving northwest, was expected to produce falls of 100mm to 150mm in the Buller-Nelson ranges.
It was then forecast to move across Taranaki, the central high country and the Bay of Plenty.
Wellington, Wairarapa and the lower east coast of the North Island should escape the brunt of the rain, Mr Downs said.
The front would be followed later in the week by a low-pressure system from the Tasman, producing unsettled, showery weather for the New Year. Its track was not yet certain.
