A man was killed and another severely injured in a boating accident on Sydney's northern beaches on Saturday.

Police said the accident happened at Newport about 2:30pm, during an organised yacht race in the Pittwater body of water.

One 72-year-old man died at the scene and another man the same age was airlifted to the Royal North Shore Hospital with severe leg injuries, where he remains in a critical condition.

Northern Beaches Police Chief Inspector Graeme Pickering said police seized the yachts that crashed and that they would undergo forensic examination.

He said he could not go into what happened during the race.

"That will be determined by the investigation itself," he said.

"At this stage its too early to give you exact numbers, but I believe there are quite a number of boats in that race.

"We'll be interviewing quite a few witnesses involved."

He said police were appealing for any members who may know about the incident to come forward and speak to police.

Chief Inspector Pickering said the witnesses he had spoken to so far were distraught.

"I spoke to a number of senior members of the club and of course they are all very upset, they would have known these people personally," he said.

"Police from Marine Area Command are conducting inquiries into the incident and a report will be prepared for the Coroner," an earlier police statement said.