A charter fishing vessel was overloaded and travelling too fast in shallow water when it struck a rock in Foveaux Strait, a report into the incident has found.
During a cod-fishing trip to Seal Rocks near Ruapuke Island in August 2012, the Torea was carrying 24 passengers and three crew members.
The Torea was permitted to carry only 20 passengers but the skipper was unaware that he had more than 20 on board, the Transport Accident Investigation Commission said in its report.
During the trip, the Torea struck an uncharted rock as the skipper was increasing speed to move to another area.
The vessel's hull was damaged in the collision and water began flooding the main cabin.
The skipper made a distress radio call, then deliberately beached the vessel on nearby Ruapuke Island to prevent it sinking, the report said.
All passengers were taken off the Torea without injury.
The commission concluded the collision with the rock was a risk associated with this type of fishing that had been reasonably mitigated by the skipper.
"In this case, however, the collision could possibly have been prevented had he remained at a slower speed until clear of the shallow area, which was known for the existence of uncharted rocks."
Carrying four more passengers that were permitted was a "safety issue", even though the Torea was carrying sufficient lifesaving equipment for the additional passengers on board, the report said.
The operator had since addressed this safety issue, so the commission made no new recommendations.
Lessons learned from the inquiry:
* skippers must know how many people they have on board and ensure the maximum permitted number of passengers was never exceeded; and
* it was risky operating small craft in shallow areas that had not been fully surveyed, so skippers must proceed with the utmost caution at minimal speed to avoid colliding with uncharted dangers.