Sydney - Five people died and three were seriously injured when a trawler collided with a smaller boat in Sydney Harbour on Thursday.

All of those killed or injured were on the smaller boat, described as a half-cabin work boat, which had its stern sliced off in the 3:00 am (1700 GMT Wednesday) accident about 150 metres offshore.

The boats have been towed to Water Police headquarters, and police have yet to interview any of the victims.

Royal North Shore Hospital spokeswoman Pat McDermott said eight men and one woman had been brought in, but six had been released.

One man was described as being in serious condition with head injuries and two people had less serious injuries.

"The ages range from 18 to 31, the youngest person is the young women," she told national broadcaster ABC. "I think they are going to be fine, but we have three that are in resuscitation who are critically ill."

Police said all the people involved had been accounted for after an air and sea search.

The collision happened at Bradleys Head, which juts out on the north shore of the harbour and has to be negotiated by boats heading east out to sea or on to Manly and other harbour attractions to the north. (dpa)