A person is dead after a boating incident in the Marlborough Sounds overnight.

Police say one person died and two people were injured after two boats collided in Okiwi Bay, about an hour's drive north of Nelson.

They say the incident happened 7.25pm.

Okiwi Bay Voluntary Rural Fire Force fire chief Ian Montgomery said details were sketchy but the collision happened where boats were moored in the harbour.

"It was very, very dark and the cloud cover was quite low last night, " he said.

Montgomery said the alarm was raised by one of those injured on board the crashed boats.

Firefighters helped transfer the most seriously injured person to a waiting helicopter.

He said he didn't know what type of boats were involved but neither sank in the collision.

"It was quite a dark night and it was a pretty busy scene," he said.

Police say the seriously injured person initially survived the on-water smash but died on the way to hospital.

Two other boaties were also injured.

An ambulance spokesman said one person suffering moderate injuries was taken to Wairau Hospital in Blenheim for treatment.

Police say they are working to determine the circumstances surrounding the marine incident.