A front arriving in Auckland and Northland this evening will bring rain, isolated thunderstorms and possibly small tornadoes.
This morning the front was bringing heavy rain to Fiordland and Westland, MetService meteorologist Michael Martens said.
A heavy rain watch was in place there with up to 90mm forecast by 1am Tuesday.
The front would move on to Northland this afternoon and evening, and lie over Auckland at night, with bands of rain and possible thunderstorms.
These thunderstorms would boost rainfall, and there was also the possibility of a small tornado, mainly near the coast.
Ahead of the front northeast winds could pick up, especially in exposed parts of Northland, Auckland and Coromandel.
The rest of the country was in for a relatively fine Monday, with just some moderate northeasterly winds.
On Tuesday and into early Wednesday a low and associated fronts would move slowly east over the country, bringing rain to many areas and strong winds in the upper North Island with a southwest change.
Heavy rain was expected about Auckland, Coromandel Peninsula, Waikato and Bay of Plenty on Tuesday, with potential for Taranaki and Nelson too.
"Most areas will see some rain tomorrow, with just the east coast from Canterbury to Gisborne staying dry," Martens said.
On Wednesday a narrow ridge would pass over the country, followed by increasing northwesterlies and another front in the South Island Thursday.
This would spread on to the North Island late Thursday and into Friday.
"The weather is looking quite changeable through the week, with normal to heavy rainfall out in western areas," Martens said.
The freezing level would lower on Wednesday ahead of the front to 800m in parts of the South Island.
"Nothing too cold, just normal for this time of year."
Cloudy periods. Rain developing in the afternoon, possibly heavy. Strengthening northeasterlies, northwest overnight. 16C high, 10C overnight.
Cloudy periods and isolated showers. Rain developing in the evening. Strengthening northeasterlies, northwest overnight. 15C high, 10C overnight.
Partly cloudy, possible morning fog. Rain developing evening. Northeasterlies developing morning. 15C high, 8C overnight.
Cloud increasing. A few showers developing afternoon, turning to rain late evening. Northeasterlies developing morning. 15C high, 10C overnight.
New Plymouth Morning low cloud or fog. Rain developing evening. Northeasterlies developing morning. 14C high, 10C overnight.
Napier Some morning low cloud or fog then mainly fine with high cloud. Northerlies. 15C high, 7C overnight.
Wellington Cloud increasing. Northerlies. 13C high, 8C overnight.
Nelson Often cloudy. A few showers. Northerlies. 13C high, 8C overnight.