Almost half the boats built to the same design as the Easy Rider have sunk, sparking an urgent safety call for the remaining vessels operating.
The Easy Rider, which capsized off Stewart Island in March with the loss of eight lives, had been found to have stability issues, the Transport Accident Investigation Commission (TAIC) said.
The boat was believed to have been built in about 1975 to the Owenga design for use in the Chatham Islands fisheries.
Constructed of steel, it was about 11 metres overall length with a beam of about 3.55 metres, and powered by a single diesel engine.
The boat was the fourth of that design to be lost at sea, the commission said today when it issued an urgent safety recommendation stemming from its on-going inquiry into the March 15 sinking.
"There appear to have been stability limitations with the Easy Rider which will be shared by other boats of the same design," TAIC chief investigator of accidents Captain Tim Burfoot said.
The commission said it had evidence that, since the Owenga class fishing vessels were built, authorities had recognised that they had limited reserve stability and could become unstable if too much weight was loaded on deck.
In one case the surveyor recommended that a plaque be placed in the wheelhouse warning skippers not to load too much on deck. The commission had made preliminary calculations which supported that view.
TAIC believed that of nine boats built to the same design nearly 40 years ago, at least five were still in service.
They wanted owners of the remaining Owenga class boats located and told of the stability problems, and Maritime New Zealand was doing that.
"The boats can be operated safely within these limitations, but owners and skippers need to know of them first to do so."
Bufoot said that while the boats' limitations had been known previously, "it may be that information has not been passed on over the years with changes of owners, skippers and surveyors for these boats".
TAIC used urgent safety recommendations to ensure matters that appeared to need attention could be addressed when they were found, rather than waiting for the issue of an interim or final report.
"I have to stress that the commission has not yet made a finding as to whether the issue highlighted today contributed to the Easy Rider accident, but it is of concern. Accidents and their impacts typically result from a range of factors coming together," Burfoot said.
Maritime NZ said it had started a process to identify all Owenga class fishing vessels to assess stability characteristics and discuss associated operational requirements with the owners of the vessels.
That process would be conducted by the end of May.
It was assessing whether there were other vessels that may have similar stability characteristics requiring attention.
Spokeswoman Sophie Hazelhurst said three owners of Owenga class design boats had already been contacted and warned.
She said two other owners were away, but their boats were docked and they would be told of TAIC's recommendation when they returned.
Hazelhurst would not reveal the names or details of the boats, saying it would be a breach of privacy.
The Commission hoped to publish a final report into the tragedy by about March 2013.
HOW THE TRAGEDY UNFOLDED
The Easy Rider had set off from Bluff in weather described by local fishermen as poor.
TAIC said the weather had been forecast to deteriorate further with the passing of a frontal system across the Foveaux Strait area.
The wind was forecast to increase to about 40 to 50 knots from the northwest before easing to 15 knots from the south west after the front had passed.
Easy Rider had been loaded with ice and bait in the fish hold, and cod and crayfish pots and other associated fishing gear on the deck, TAIC said.
The skipper then loaded stores, equipment and personal effects for the passengers who were going muttonbirding.
The stores and equipment covered most of the back fishing deck and in places was stacked as high as the roof of the wheelhouse.
Easy Rider left Bluff about 8pm on March 14 for Great South Cape Island off the west coast of Stewart Island.
About three minutes after midnight, north of the Bishop and Clerks Islands at the western end of the Foveaux Strait in an area known for strong variable currents and turbulent water, Easy Rider was engulfed by a large wave and capsized, remaining afloat for about two hours before sinking.
The sole survivor, crew member Dallas Reedy, was sitting out on deck at the time. He described the wave swamping the deck and the vessel heeling violently to port and capsizing, TAIC said.
Four bodies were found - Shane Topi, 29, Boe Pikia-Gillies, 28, John Karetai, 58, and Peter Pekamu-Bloxham, 53. Still missing are skipper Rewai Karetai, 47, Paul Fowler-Karetai, 40, David Fowler, 50, and Odin Karetai, 7.
Jill Karetai, Easy Rider skipper Rewai Karetai's aunt, welcomed the TAIC findings.
"Anything that helps to save further lives and educates people, is a great help,'' she said.
She said it was not known if the skipper and crew on board Easy Rider were aware of stability issues.
"That is one thing we are waiting to find out at the inquest. There has been terrible speculation and the sooner it is sorted the better it will be for the families,'' she said.
The inquest into the deaths would begin in Invercargill on June 6, before chief coroner Judge Neil MacLean.
Justice Ministry spokesman Steve Corbett said the limited-scope inquest was expected to conclude in one day, although a reserve day had also been set.
The inquest would look into limited details and await the TAIC report, Corbett said.
Any safety recommendations would be welcomed, Reedy said.
He would attend but not appear as a witness.
Reedy said yesterday that safe practices and procedures may be able to be tweaked but there was not much that could be done.
"I don't know what more they can do. The Titanic sunk and it was unsinkable ... but any recommendations to make it safer, I'm all for it," he said.
The inquest would also bring closure for the families, he said.
Jill Karetai said the inquest would answer the families' questions "what actually happened?".