air and sea search is underway for a man in his 60s who is missing after a tugboat that was towing a barge capsized in Gladstone Harbour.

Three other crewmen have been rescued, emergency services say.

Police were alerted that a boat had turned over in the Gladstone Harbour near to the mouth of the Calliope river around 1pm (AEST).

There were four men on board the stricken vessel and three were rescued - however the skipper of the boat is still missing.

Police, volunteer marine rescuers, coastguard and the CQ rescue helicopter are scouring the area for the missing man.

Maritime Safety Queensland general manager Patrick Quirk says a search for the missing man is under way.

He said the tug was towing a barge in Gladstone Harbour on Saturday afternoon when it sank near a coal terminal.

The barge did not sink.




Mr Quirk said it was not known what caused the tugboat to sink and a full investigation would be launched but for now the focus is the search for the missing crewman.

All four were onboard the 23-metre tugboat when it capsized about 1pm.

Gladstone water police rescued three of the men and continue to search for the fourth.

A rescue helicopter has also joined the search and a 100m exclusion zone has been set up around the sunken vessel.