Strong winds that lashed the top of the South Island overnight dumped a catamaran in the middle of the road.

In the Nelson suburbs of Tahunanui and Monaco firefighters responded to reports of sea surge but were forced to continue on foot when they found Martin St blocked by a catamaran, Fire Emergency NZ southern communication's Ian Littlejohn said.

The storm caused power cuts on the West Coast, Nelson, Marlborough and parts of the South Island east coast early on Wednesday morning, he said, but no one was injured.

Freedom campers at Mapua, near Nelson, had to be helped away from the waterfront.