A red rain warning has been issued for Westland as MetService expects 600mm to 800mm of rain to accumulate about the ranges, and possibly more in localised areas, and 100mm to 200mm about the coast.
The heaviest rain is expected from late Friday morning where peak rates could reach 30 to 35mm per hour.
The warning will be in place till Saturday at 8am.
MetService said the long duration of heavy rain was expected to be especially impactful for Westland south of Otira.
It could cause dangerous river conditions and significant flooding, the weather agency said.
"Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities."
MetService meteorologist John Law told RNZ they had issued the red warning due to heavy and continuous rain.
"In the mix as well will be some thunderstorms, which will bring those localised heavy bursts of rain as well, so well worth keeping an eye on the forecast.
"We are looking at a lot of warm moist air being dragged in from the north and the west, that contains a lot of that moisture as we go through the rest of the today.
"Then another front moves in from the Tasman Sea as we go through Friday and that brings another belt of heavy rainfall."
Orange rain warnings are in place for the Grey District and Fiordland until Saturday morning and Friday night, respectively.
Earlier before the red rain warning, Law told Morning Report they were expecting 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.
"[We] could find streams and rivers rising and perhaps some surface flooding."
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 were 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."
