Maritime New Zealand is trying to contact the owners of four boats of the same design as one which sunk in March, killing eight people, after investigators found it had stability flaws.

The Easy Rider capsized in Foveaux Strait with nine people on board about midnight on March 12.

One person was rescued after 18 hours in the chilly waters but the other eight perished.

An urgent safety recommendation from the Transport Accident Investigation Commission (TAIC) says owners of boats from the Owenga class, of which the Easy Rider was one, need to be aware the boats have stability issues if too much weight is loaded on the deck.

A Maritime New Zealand spokesman told NZ Newswire it believed there were still four Owenga boats sailing.

Three were in commercial use in New Zealand, and one was believed to be in Tonga.

Some owners been spoken to about the stability problem and attempts were being made to contact the other owners.

Investigators found the stability flaws when carrying out tests on the Owenga class to see what they could hold, and also discovered authorities had previously raised similar concerns.

The Easy Rider is the second of the Owenga boats to have suffered a fatal sinking.

Timaru fisherman Peter Hunter died after his trawler, the Journeyman, sunk in late November last year.

A TAIC spokesman said the boat had never been found and it had not been able to discover whether its sinking had anything to do with stability problems.

Early investigations into the Easy Rider sinking showed that gear covered most of the aft fishing deck and in places was stacked as high as the roof of the wheelhouse, TAIC said.

"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," TAIC chief investigator of accidents Captain Tim Burfoot said.

Capt Burfoot said the stability issues didn't mean the boats were unsafe to use provided those using them were aware of the issues.