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.”
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, meteorologist John 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.”
