Auckland was in for a relatively fine Saturday with a high of 17C, before it would all turn to custard on Sunday about midday when wicked winds and heavy rain would kick in.
The hardest-hit places will be Fiordland, the Canterbury lake and river headwaters and Westland south of Otira, where heavy rain warnings are in place to Sunday morning with more than 100mm of rain forecast. There was also a moderate risk of thunderstorms in these areas.
The heavy rain could cause streams and rivers to rise rapidly, and lead to surface flooding and slips causing hazardous driving conditions.
Gale northwesterlies were forecast for parts of the South Island high country today and into the early hours of Sunday.
As the front moved north overnight Saturday, heavy rain would spread to Nelson, Buller and north Westland, before crossing the Cook Strait and dousing the Tararua Range and Taranaki to about 2pm Sunday.
Gale northwesterlies could also hit Marlborough and Wellington until about 1pm Sunday.
James said similar "unsettled" weather could be expected into next week.
"We've shifted into an unsettled pattern, with another feature arriving on Tuesday. We can expected rain to on and off for a little while."
Today's weather
Whangārei Fine, evening cloud. Northerly breezes. 19C high, 13C overnight.
Auckland Fine, apart from isolated morning and evening showers. Northwest breezes. 17C high, 12C overnight.
Hamilton Cloudy periods, chance shower. Northerly breezes. 17C high, 10C overnight.
Tauranga Cloudy periods. Northwest breezes. 17C high, 10C overnight.
New Plymouth Cloudy periods and one or two showers. Northwesterlies. 16C high, 12C overnight.
Napier Fine. Northwesterlies. 19C high, 11C overnight.
Whanganui Mostly cloudy, morning shower or two. Northwesterlies. 17C high, 11C overnight.
Wellington Fine, then showers developing this evening. Northerlies strengthening. 14C high, 11C overnight.
Nelson Fine with high cloud, then showers towards evening. Northerlies strengthening. 15C high, 11C overnight.
Christchurch High cloud at times. Northeasterlies strengthening. 16C high, 9C overnight.
Dunedin Cloudy at times, a few spots late rain. Northeasterlies. 14C high, 8C overnight.