The Transport Accident Investigation Commission says there could be dozens of other boats with stability problems similar to the Easy Rider which capsized in Foveaux Strait.
The vessel went down off Stewart Island on 15 March this year. One man survived, four bodies were later recovered and four others, including a seven-year-old boy, have not been found.
Three other boats of the same design have previously sunk.
On Tuesday, the commission issued an urgent safety recommendation for four Owenga class boats made in 1975 which are still in operation.
Maritime New Zealand is working to contact their owners and skippers to warn them how easily the boats could be overloaded and capsize.
The authority says it has met one owner, another is away muttonbirding, a third has yet to be spoken to, while another boat is thought to be in Tonga.
TAIC's chief investigator Tim Burfoot told Radio New Zealand's Checkpoint programme on Tuesday the overloading problem could be more widespread, but the commission is not sure how many boats may be affected.
Jill Karetai, whose nephew skippered the Easy Rider, believes the boats should be taken off the water immediately.
But Captain Burfoot says he does not think Maritime New Zealand will order them to stop sailing. He says the design of the boats is not the problem, but the way they are loaded, and owners and skippers need to be aware of this.
"All we are saying is, there are limitations in what they can be used for, just like any other boat. You can overload or load in such a way any vessel that will cause it to capsize or sink. Some will take more than others to do that."
The Transport Accident Investigation Commission intends to publish a final report by March next year.