Substantial evacuations are underway in Tasman as the region is smashed yet again by torrential rain

At least three people are stranded in flood waters, while evacuations are taking place in areas already impacted by rain two weeks ago.

Meanwhile power is out across multiple locations including Golden Bay, Motueka, and Collingwood - and is expected to remain so overnight.

Nelson Tasman Civil Defence Controller Alec Louverdis says helicopters are attempting to get to those stuck.

He says unfortunately it's getting late and dark, and they may have to stay overnight if the choppers can't reach them and they're confirmed safe.

Louverdis says teams are working on alternative arrangements if safety becomes an issue.