Parts of the South Island can expect to get drenched today, while an unusually warm weather system will push temperatures to 25 deg C and over in Christchurch.
A heavy rain watch is in place for the headwaters of the Canterbury lakes and rivers, with the rainfall set to exceed warning criteria within 20km east of the main divide.
MetService has also issued heavy rain warnings for Fiordland and around the headwaters of the Otago lakes.
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 would close from East Gate (Hollyford) to Donne River (Westgate) at noon today due to intense rain heightening the risk of avalanche.
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.
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
The first warm system of year is being fed by snow melt, Loots said.
Christchurch is set to reach a high of 26 deg C today, then 21 deg C on Thursday and Friday. At 11pm today it was already 21 deg C in the city.
Rivers are still running high further south as eastern Otago continues to be affected by the flooding a few weeks ago.
Two factors made this weather system 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. It was likely the watches would be upgraded to warnings, Loots said.
Dunedin and Oamaru are also set to reach 24 deg C today.
- Additional reporting ODT Online
