Roger King and his crew of three were last night resting at their Hobart homes after the Anmaropa capsized on the notorious St Helens barway yesterday morning.

Mr King has been hailed as a hero after his rescue effort, but authorities have said the vessel should not have attempted to cross the treacherous barway against advice.

Inspector Shane LeFevre, of St Helens police, said it appeared that after the vessel had called for assistance, it did not follow advice to wait for a rescue vessel to guide it into the bay.

"With such weather conditions like a three or four-metre swell, combined with a low tide, it (the barway) is extremely dangerous to cross," he said.

Mr King defended his actions last night, saying he had to make a quick decision to either wait out in deteriorating weather conditions or test his luck getting to Triabunna, 11 hours away.

"Waiting would have prolonged the risk and we took every precaution - we checked the weather ... and it had been raining heavily before, which had flattened the sea out a bit," he said.

It was the crew's sixth trip across the St Helen's barway this season and the trawler was carrying 8.5 tonnes of scallops to unload at the town's port.

Mr King and another crew member were manning the helm while the other two men were in the crewhouse helping with navigation as they approached the barway.

The boat became caught in the shallows on the bar and soon after was knocked sideways by a wave.

"The crewman who was with me was washed clear and I got sucked into the wheelhouse by the flood of water," Mr King said.

"There were three of us trapped in the wheelhouse as the boat sank on us - basically like rats in a jar.

"We were stuck there for a couple of minutes and we were covered in diesel that was leaking out of the tanks and coming in with the water flooding the wheelhouse - so we were inhaling fumes and choking on diesel."

Mr King located the front hatch and forced it open against the pressure of water that filled the room.

He and another crew member managed to swim to the surface and tried to hold the side of the boat with a third crewman.

They noticed that the fourth member of the crew, Joe Jacobson, was still stuck underwater.

"I managed to get hold of a piece of timber and smashed the wheelhouse window and pulled him out," Mr King said.

"We could hear him yelling inside the wheelhouse `get me out, get me out'."

The four men were rescued by the police vessel Relentless and rescue vessel Break O'Day and were taken to St Helens Hospital to be treated for mild hypothermia and for lacerations.

All were discharged by the afternoon and returned to their homes in Hobart.

Mr King believed the sand- removal program, recently completed on Blanches Beach to make the barway more navigable, had made it more unpredictable.

"All that has done in the last three weeks has moved the channel a bit and made it more hazardous because it's rapidly changing now," he said.

"Quite frankly it's a disgrace - the lack of maintenance and infrastructure at that port."