Photo: Supplied / MetService
A red rain warning has been issued for Westland on the South Island's West Coast.
MetService is expecting 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.
Sandbags available
At least 500 sandbags are being made available for Hokitika residents with heavy rain forecast.
Westland District Council said sandbags should be used by businesses and residents in areas likely to be worst affected by surface flooding and cars causing bow waves.
Operations manager Erle Bencich said from previous experience, the most affected areas were likely to be Revell Street along the beachfront, Tancred Street and Sewell Street.
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."
Motorists planning to travel along the West Coast are being told to check conditions before they set out.
The New Zealand Transport Agency / Waka Kotahi said sections of the state highway network might be closed overnight Friday if driving conditions became unsafe.
The agency said it was monitoring conditions closely and receiving regular updates from MetService.
Emergency management plans
Emergency management on the West Coast is mobilising staff and resources in preparation for heavy rain over the next two days
While the focus is on Westland, a close watch is also being kept on the Grey District which is under an orange heavy rain warning.
There are concerns the rain will cause slips and flooding which are likely to make some roads impassable, potentially isolating communities.
Emergency management agencies and first responders have been meeting for a briefing on the situation.
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."
"It's an incredibly large amount of rainfall in an already wet area" - John Law
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."