A number of residents in the West Coast's Buller District have been asked to voluntarily evacuate as heavy rain causes the local river to dangerously swell.
Residents in low-lying areas of Westport should evacuate before nightfall, says West Coast Civil Defence Emergency Management Group (CDEM).
An emergency alert issued shortly before 4:30pm urged anyone living on Gladstone St, Russell St intersection and nearby areas to voluntarily evacuate, as well as those living on Colvin St and Rintoul St, Menzies St, Roebuck St, Bentham St, Disraeli St, and in the Westport Domain area.
"If you see rising water do not wait for official warnings," Civil Defence said in the alert.
"Head for higher ground and stay away from floodwater."
Buller District Council has already declared a State of Emergency for the area, with officials saying local houses are likely to flood on Friday night. A rare 'red warning' has been issued for parts of the South Island, indicating the strong winds and heavy rain are likely to impact communities.
Buller Mayor Jamie Cleine said people with properties prone to flooding should make plans to evacuate now.
He said the Buller River is currently running at a 50-year flood level - and it is tracking towards a one-in-100-year event.
"The key risk area is overnight, so with the river already running high, we've got another 200-280mm of rain forecast in the next 36 hours - so it's highly likely that the river will continue to rise."
MetService issued the 'red warning' - only the third to be issued since 2019 - as the heavy rainfall could cause significant flooding and slips from central Westland to Buller.
"A red warning is in force for significant heavy rain from central Westland to Buller. Flooding is likely, as are slips and disruption to transport. Some communities could become isolated, and livestock should be moved to high ground," MetService said.
It also said up to 400mm of rain in the ranges, and 200mm on the coast, is expected to fall through to Saturday morning.
Westland Mayor Bruce Smith is advising locals to prepare for the possibility of evacuation. Earlier, he warned residents to be wary of flooding and slips over the next 24 hours and that power outages were likely.
"Make sure you're prepared and make sure your families have got their Civil Defence pack there, we're hoping it won't be needed but preparation, preparation, preparation."
The Waka Kotahi NZ Transport Agency says Friday's torrential rain has caused extensive surface flooding, slips and highway closures.
Journey manager Tresca Forrester is encouraging people to stay off the roads due to hazards such as visibility issues and potential slips.
"Rain is forecast to continue overnight so we encourage people to avoid driving in these conditions."
Arthur's Pass has closed between the West Coast and Canterbury and is unlikely to open before Saturday, meaning motorists will need to detour via Lewis Pass.
State Highway 6 has surface flooding in places between Hokitika and Haast and is closed at Inangahua due to high river levels.
As more heavy rain is forecast into Saturday, motorists should be aware that State Highway 3 and 6 could close with little notice. Key local roads could also flood and become impassable.
In a statement on Friday evening, Fire and Emergency (FENZ) said it is ready to support Civil Defence on the West Coast and in the Nelson-Tasman region if weather-beaten communities require assistance.
Crews in Westport have been helping with precautionary evacuations of residents in low-lying areas of the town, Response Coordinator Mike Bowden said.
FENZ activated its regional coordination centre in Christchurch earlier on Friday and an Urban Search and Rescue squad has arrived on the coast.
"Fire and Emergency is front-footing support for the West Coast and is ready for potential escalation anywhere in the upper South Island," Bowden said.