The girl, understood to be the daughter of the skipper, was caught under the hull as it overturned during an attempt to cross the Moruya River bar at 7am on Saturday.

Three men, aged 46, 29 and 18, and two other 13-year-old children were thrown from the boat in the incident and rescued a short time later after nearby vessels raised the alarm.

Police understand no one in the boat was wearing a life jacket and have stressed that jackets should always be worn when crossing a coastal bar.

The five survivors were taken to hospital to be treated for hypothermia, while one of the children suffered a foot injury.

A lifesaver attending the incident was able to swim underneath the boat and pull the girl out from under it, according to police.

Acting Superintendent Darren Wood said the 13-year-old had been trapped underwater for between 20 and 40 minutes before she was pulled free,

"I believe she had been under the vessel for an extensive period of time ... so there was no resuscitation," he said.

Earlier reports from the scene suggested emergency services had attempted to resuscitate the girl.

Supt Wood said the boat had been trying to cross the bar during an outgoing tide, which he said was the most dangerous time to attempt it.

"When you do cross a bar on an outgoing tide, the waves will become steeper and closer together therefore making any bar crossing at an outgoing tide the most difficult time to cross," he said.

Lifesavers from Moruya, Broulee and Batemans Bay Surf Life Saving clubs came to the aid of the Canberra group after receiving an emergency call-out from a nearby boat.

Four were pulled from the water and returned to shore on their jet ski, with the fifth rescued by boat later.

"The skipper of the boat was the father of the girl who was deceased," SLS duty officer Donna Wishart said.

"It's absolutely tragic. Our hearts are with the family and our lifesavers themselves have been offered counselling."

Ms Wishart said the boat had been heading out to sea in rough conditions and she didn't believe any of the group were wearing life jackets.

"There was up to two-metre swells along that coast today and there has been hazardous surf warnings in place for the last few days," she said.

The group were family and family friends who had been staying at a nearby holiday house according to police.

Supt Wood said the boat was a 6.3 metre fibreglass half cabin and one of the men involved had more than 30 years boating experience.

"We would always send out the message that you must be wearing a life jacket when you cross a coastal bar and that the best time to cross that bar is on an incoming tide or at the change of tide," he said.

Australian Associated Press