Three men and three children were in the 6.3-metre vessel when it overturned while attempting to cross the Moruya River bar at about 7am on Saturday.

The girl was trapped underwater for up to 40 minutes in the hull of the overturned boat, according to police, who say they are investigating the circumstances of the incident.

The girl is understood to have been the daughter of the skipper.

Police said none of the six people on board were believed to have been wearing life jackets.

The boat was crossing the Moruya River bar when it was hit by three consecutive waves which caused it to capsize.

Acting Superintendent Darren Wood said the boat had been crossing 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.

Australian Associated Press