West Coast Civil Defence has set up an emergency operations centre in Hokitika as a precaution, with heavy rain forecast for the region this weekend.
MetService says up to 200mm will fall in Westland north of Otira and up to 150mm in the Buller District.
Emergency Management Group Controller John Canning said the modelling showed the system moving north over the day.
He said another emergency operations centre will open in Greymouth this evening [Saturday] to keep an eye on things.
Two days ago the forecast for the weekend was terrible, but it has since eased somewhat.
The forecaster warns heavy rain could cause rivers and streams to rapidly rise, and surface flooding was likely.
Canning said 15mm of rain an hour is expected in Greymouth this afternoon, and it was worth being prepared.
"It's fairly changeable but we're not having heavy rainfall at present so we're gearing up, if it does get heavier, for flooding in the townships on the coast. And hopefully we can deal with that.
Resident's and drivers should take care for slips and surface flooding on roads, and keep an eye on the West Coast Civil Defence's Facebook page for updates.
