Not enough lifejackets has been identified as a key reason eight people died when the charter fishing vessel Francie foundered in rough conditions.
The boat, with a skipper and 10 passengers on board, was trying to cross the Kaipara Harbour Bar on November 26, 2016, after fishing off the coast.
By the time the group tried to re-enter Kaipara Harbour, the ebbing tide and a developing swell had combined to cause steep waves, breaking in several directions.
A Transport Accident Investigation Commission report, released this morning, said the Francie was struck from behind by a large breaking wave as it tried to cross the bar, causing it to roll heavily, before capsizing and sinking.
Witnesses and Coastguard crew who helped in the search and rescue operation estimated waves were 4-8m high.
Only three people on board were wearing lifejackets when the boat capsized. Eight of the 11 people died.
The commission found conditions at the bar were predictably unsuitable for the Francie to cross, and noted the skipper was known to cross the bar when conditions were too rough.
Its report said the skipper showed people where the lifejackets were when they came on board, but didn't require passengers to put them on.
Three of the people had brought their own lifejackets, and were wearing them. The skipper and other seven passengers were not wearing lifejackets when the boat departed.
The report says it is "virtually certain" lives could have been saved if lifejackets were worn properly, including crotch straps to stop them riding up and slipping over the wearer's head.
One of the survivors stayed afloat in the rough waters only by finding two of the Francie's lifejackets floating nearby. They were too small for him, but he put an arm through each neck hole, and used a piece of timber nearby to hold himself above the waves.
One of those who died had been wearing his own lifejacket, but it was found separate from his body. It did not have a crotch strap.
Chief investigator Captain Tim Burfoot said skippers should always exercise extreme caution when crossing harbour bars because sea conditions could change for the worse in a very short time.
"It's very likely that only three of the 11 on board were wearing lifejackets at the time, and no one was wearing a lifejacket provided on board in accordance with the Maritime Rules.
"It's virtually certain that they would all have had a better chance of survival had they all worn lifejackets.
"They would have had an even better chance if their lifejackets had been fitted with crotch straps to prevent riding up."
The commission has recommended commercial vessels carry lifejackets to suit the vessel's place and time of operation, and that crotch straps be fitted to lifejackets on commercial vessels that operate out of bar harbours and on exposed coastlines.
It also recommends recreational users be encouraged to fit crotch straps to lifejackets.
It also found there were problems with raising safety concerns, because there is no dedicated, formal process within Maritime NZ to report them.
"The skipper of the Francie had a propensity to accept a high level of risk when deciding whether to cross the Kaipara Harbour Bar," Burfoot said.
"Others in the industry and community knew this, but it wasn't reported to authorities."
It has recommended that Maritime NZ sets up a way to report maritime safety concerns, and ensure the method is advertised to the public and maritime community.