MetService has upgraded its heavy rain watch for Buller and Westland to an orange warning.
The warning covers Buller and the Nelson Lakes, Westland, and the headwaters of the Canterbury lakes and rivers north of the Rangitata River.
MetService expects 160 to 200 millimetres of rain to fall in Westland and 150 to 200 millimetres in Buller today, mainly about the ranges.
Buller and Westland were hit by torrential rain last week, forcing people in flood-prone parts of Westport to evacuate.
There is also a heavy rain watch in place for the Tasman district, west of Motueka, from early tomorrow morning.
MetService meteorologist Ashlee Parkes said an "atmospheric river of moisture" was pushing into the West Coast.
"This heavy rain event is looking like a typical heavy rain event for the West Coast of the South Island, but because the ground is already saturated and rivers are already running high, this additional rain might cause further flooding and slips," she said.
"At the beginning of February with our red heavy rain warning in the area, we saw over a metre of rain in the ranges of Westland. In this orange event, we're expecting about 200-240 millimetres of rain in the ranges, so not nearly as much, but given the ground is already wet, we are watching the situation."
The final piece of equipment for a new flood monitoring system is now in place in Buller River, which will give up to eight hours' evacuation warning.
Buller District Council staff are still assessing damage to infrastructure from last week's severe weather.
State Highway 67 between Westport and Karamea remains down to one lane due to a slip.
State Highway 67 pictured during heavy rain last week. Photo: RNZ / Niva Chittock
A slip that damaged the Lake Rochfort Dam also washed out the Christmas Creek bridge.
The Insurance Council of New Zealand says reports of damage to insured businesses and homes were not widespread.