A desperate search is underway in Sydney's south tonight for a man who is missing, after a speed boat crash on the Georges River at Revesby.

Two men were seen doing laps along the river this afternoon, with witnesses saying they may have been testing out a new speed boat.

It's believed the pair were travelling at high-speed, and somehow the boat flipped near some mangroves, possibly after hitting a pontoon.

The two men went under the water, but only one resurfaced. He has been taken to St George Hospital, where he is being treated for what are believed to be minor injuries.

The other man's parents arrived on scene a short time later, where they were counselled by police.

A large group of friends also gathered by the river, with one so desperate he jumped into the river and tried to swim out to help locate his friend.

7News has been told the two men are part of a big boating community.