It could be another 18 months before investigators can say how the grounding of the Aratere in June might have been prevented.
The interim report from the Transport Accident Investigation Commission, released on Thursday, said the crew of the Cook Strait ferry did not know how to turn off the autopilot when they realised the ship was heading towards the shore north of Picton.
A final report is expected to make recommendations - but TAIC said it was a complex investigation, and it would probably not know until 2026 when that report would be ready.
The interim report's facts were not disputed by the Merchant Service Guild - the union representing ship masters and officers.
"It's a very thorough report, there's nothing we didn't know about it already, from what we've heard from our members, and they've covered all the major points," vice president Iain MacLeod said.
The accident was "incredibly serious", but MacLeod was confident that Interislander had made the necessary changes to stop it happening again.
Interislander executive general manager Duncan Roy said no one in the organisation knew that in a "specific set of circumstances" - where the rudders were out of sync with the steering wheel - a "five-second override" of the autopilot was needed.
A new Kongsberg steering system had been installed just weeks prior to the grounding.
Interislander was working with Kongsberg to understand why it did not know about the critical change to the autopilot system, Roy said.
But everyone on the Aratere bridge now knew how to use the system, and they were tested on it, he said.
RNZ has approached Kongsberg for comment.
The Aratere grounded in June. Photo: RNZ / Angus Dreaver
Meanwhile, Pete Beech, the chair of lobby group Guardians of the Sounds, said the accident showed why ferry operators should use the northern entrance to the Marlborough Sounds, rather than the narrow Tory Channel.
The group had long been campaigning for the northern entrance to be the standard route, claiming accidents were more likely in the Channel, risking lives and environmental damage.
"When you're in a narrow channel, you have very little time to fix your problem, to change your course, to take evasive action," Beech said.
"If they're in the northern entrance, it's a big, wide road."
The northern entrance added about half an hour to the journey across the Strait, he said.
"Both Bluebridge and KiwiRail, whenever ... they're carrying an injury, whenever they have a mechanical problem, they use the northern entrance, because it is much safer.
"So they've already set the precedent."
He hoped that would become the standard route, once TAIC had completed its investigation.
"They really dodged a bullet that day, if that had happened when they were going out through the heads of Tory Channel, it could have been an absolute disaster."
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