Their vessel, which was loaded with scallops and heading to unload in St Helens, remains overturned on the barway.

Rescue crews arrived to the scene sometime after 8am to find two crew clinging to the overturned hull of the boat, with waves breaking over it.

St Helens Marine Rescue Association operations manager Greg Schmerl said two were trapped in the wheel house.

``They're very lucky – they were under water for a little while,'' he said.

The two men received immediate medical attention when brought back to land art Akaroa, outside St Helens, for shock and exposure and minor lacerations.

The scallop crew had requested St Helens Coast Guard by radio to escort them over the barway just before 8am, which was referred onto St Helens Marine Rescue Association.

``We had to go there and have a look but we probably would have told them to wait three hours for the tide to get full over the bar and then have a try,'' Mr Schmerl said.

``It's full of scallops so it would have been weighing a bit.''

Both the rescue vessel Break O'Day and police vessel Rentless attended the scene. Police reported that 2-3 metre seas were breaking over the barway at the time.

The vessel remains overturned on the barway, showing one metre of its keel.

The boat was fully laden with scallop catch and was coming to St Helens to unload.

Mr Schmerl said there was debris in the water, and a crew would go to the spot this afternoon to assess the likelihood of an oil spill from the ship's diesel tanks.