Parts of the South Island can expect to get drenching today, while unusually warm weather will push temperatures well into the 20°Cs in the east.
MetService has heavy rain warnings in place for Fiordland and around the headwaters of the Otago lakes.
A heavy rain watch is in place for the headwaters of the Canterbury lakes and rivers, with rainfall set to exceed warning criteria within 20km east of the main divide.
A rain watch further up the West Coast starting from tonight will last through to the early hours of Friday.
Meteorologist Karl Loots told RNZ's Morning Report programme people in Fiordland north of Doubtful Sound could expect the rain to start from this morning with as much as 250mm to 350mm on the way.
"This is very significant heavy rain, even for parts of the country that are quite accustomed to heavy rain."
The Milford Road closed from East Gate (Hollyford) to Donne River (Westgate) at noon today due to intense rain heightening the risk of avalanche to moderate.
State Highway 94 would also remain shut tomorrow and re-open on Friday, depending on avalanche risk and potential clearance works, Milford Road Alliance advised.
More rain and possibly snow near the Homer Tunnel was forecast for Saturday.
For the headwaters of the Otago lakes and rivers a heavy rain warning is in place with the heaviest rainfall forecast for Thursday.
In the 24 hours from 4pm today, 175mm to 250mm of rain was set to fall near the Divide, and 100mm to 150mm within 20km farther east.
Peak rates of 20mm/h to 30mm/h were expected tomorrow about the Divide, with thunderstorms possible.
Warm in the east
It was a warm system for this time of year and it would be fed by snow melt, Loots said.
Rivers were still running high with eastern Otago affected by flooding in some parts a few weeks ago.
Two factors made it significant.
"It's a very warm air mass being driven from the sub-tropics down to the southern parts of the South Island and then secondly, this front is moving extremely slowly so it would be a very prolonged period of rainfall."
It could last from 24 to 30 hours for many areas and could spread up the West Coast and it was likely the watches would be upgraded to warnings.
Dunedin and Oamaru were set to reach 24°C, while Christchurch has a high of 26°C.
- Additional reporting ODT Online
