Passengers on a Cook Strait crossing had a white-knuckle ride this morning after a truck's trailer unit crashed through the ferry's barriers in rough seas.
Bluebridge has launched an investigation, and says nothing like this has happened before.
Video footage from Severin D'armancourt, who was on board the ferry.
The two linked trailers containing frozen pork fell off the Straitsman near Wellington Heads on its sailing from Picton to Wellington, despite being secured by storm lashings.
Bluebridge said tonight that one of its tugs, a small tractor-like vehicle used to tow b-trains onto the ship, was attached to the trailers when they went overboard.
The tug would have sunk immediately, Bluestone says.
The ferry docked in Wellington about 11am, and Maritime New Zealand inspectors were also investigating.
French tourist Severin D'Armancourt said it had been a rough sailing, describing it as "crazy".
Initially the sea was calm but the waves became bigger and bigger and at one point it felt like the ferry was on a 45-degree angle, he said.
He didn't see the containers go overboard but said at first people were not sure whether it was a piece of cargo or a person who had fallen off.
"A member of staff say to another member of staff, 'Oh, there's something fell off the ferry', and [afterwards] a passenger going in the cabin and say, 'Oh, there's a big truck fell off of the ferry'."
Mr D'Armancourt said people were throwing up and children were crying, and it was a frightening experience. "I've took several ferries, but this one was the worst one."
Bluebridge general manager commercial Ed Menzies said in a statement it was unclear why the trailer moved and broke the railings.
Vehicles and freight were lashed to the freight deck on every sailing, he said.
No crew were injured and no-one was on the freight deck at the time.
The operator has cancelled its next sailing and put people on its rival, the Interislander.
Maritime New Zealand has issued a navigational hazard warning, saying the containers could be semi-submerged.
Wellington Harbourmaster Mike Pryce said they could cause a problem if they hit smaller boats.
The containers could end up at Lyall Bay or Baring Head in Wellington, he said.