Parts of the South Island could be set to see over a month's worth of rainfal in a day, MetService says.
A heavy rain warning is in place for Westland north of Haast, and the Grey District until 3am on Friday.
MetService expects 230 to 280 millimetres rain to accumulate about the ranges about and south of Otira, with 100 to 150mm about the coast and ranges north of Otira.
Meteorologist John Law told Morning Report that was over a month's worth of rainfall in some of those areas.
"It's an incredibly large amount of rainfall in an already wet area."
Law said it will be area to watch out for.
"[We] could find streams and rivers rising and perhaps some surface flooding."
A heavy rain watch is also in place in Buller and west of Motueka until 8pm and 6pm on Thursday, respectively.
Heavy rain watches will also come into force on Friday in Westland south of Haast, Fiordland and the headwaters of Otago lakes and rivers.
In the North Island, Auckland has been put under a heat alert with temperatures reaching a high of 28C.
Law told Morning Report the heat alerts are in place largely due to warm overnight and day temperatures.
"Night-time temperatures [will be] not much lower than 19C right the way through to about Tuesday next week meaning there's no real respite from that heat of the day."
RNZ
