A trailer containing chilled goods broke loose and crashed into the sea during a rough Cook Strait crossing this morning.

Exclusive footage obtained by the Herald shows the trailer slipping under the water after slamming though the railings of the ferry.

An eyewitness said there was a "big snap" when the trailer broke from its chains and slid toward the ship's barriers.

Aucklander Jordan Matchitt, 26, was with a friend on the ferry's top deck watching the waves hit the ship when he first noticed the truck swaying.

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"We'd been watching it earlier, in the smaller waves, and you could see the truck swaying side to side, but it was chained down and we didn't actually think it would go anywhere."

Then the seas become wilder, and Matchitt became seasick.

"Then all of a sudden, when we started turning, one big wave hit us side on, then we heard a big snap, looked back, and the truck had started to move and had slid toward the barrier."

Matchitt said, "it went back down to the other side of the waves, then slid back, hit another wave, then the trailer went off through the barrier, and it ripped the other trailer and truck off and snapped the second chain."

The sound of the trailer breaking through the barriers was like "metal smashing through metal," he said. "Everyone was just running to have a look... everyone was looking around, just yelling and screaming," he said.

"As soon as I could get my phone out of my pocket, because my hands were frozen, I took a couple of photos, then started filming, but by that stage it was already in the water."

A minute later, an announcement came over the loud-speaker asking people on the decks to move inside.

Others on the ferry described chaotic scenes amid the wild seas.

Mariia Hakala said the sea was rough when the trailer went overboard.

Mariia Hakala, 27, was travelling on the Bluebridge ferry when a truck fell overboard. Photo / Solbin Kang
Mariia Hakala, 27, was travelling on the Bluebridge ferry when a truck fell overboard. Photo / Solbin Kang

"There were a lot of waves. There was a side wave and it made the boat go sideways. The next thing I knew, people were saying a trailer had fallen into the water."

Hakala was sitting near the kitchen when it happened. "I could hear a lot of stuff falling down in the kitchen," she said. Meanwhile, passengers were vomiting, she said.

"It was really hard to walk inside because the waves were really high. People were throwing up."

Maritime NZ officials inspecting the damaged onboard the Bluebridge Cook Strait ferry Straitsman after a truck tumbled off it's deck. Photo / Mark Mitchell
Maritime NZ officials inspecting the damaged onboard the Bluebridge Cook Strait ferry Straitsman after a truck tumbled off it's deck. Photo / Mark Mitchell

She said crew gave the passengers paper bags and ice cubes.

The ship, which had left Picton at 8am, continued on to Wellington despite the incident.

Strait Shipping general manager commercial Ed Menzies said they were not sure how the trailer came off. "The unit was storm lashed and it is unclear as to why it moved and broke the ship's railing.

"The company has launched an immediate investigation into what happened," he said.

"Strait Shipping has not had an incident of this nature before."

This afternoon's sailings of the Straitsman have been cancelled.

MetService meteorologist Peter Little said gale force winds had ripped through Cook Strait this morning, causing choppy seas.

"We've had a gale there, and there's been waves of four-and-a-half metres, and up to about seven metres occasionally."

Little said the extreme conditions weren't unusual for the strait.

"It does happen from time to time, the large waves are more unusual than the gale but we do get them a few times a year in the Cook Strait."

Strait Shipping clarified that what was lost overboard was a B-train made up of two refrigerated trailer units, along with the ship's tug unit - a mini-truck which tows the trailer unit on board.

A Maritime New Zealand spokesperson said while it was likely the trailers - which were carrying chilled pork - and tug had sunk in deep water, a coastal navigation warning has been issued for an area approximately 7.5km south of Island Bay, advising of the potential for semi-submerged debris.

As we kick off the second week of the school holidays we have more windy westerly quarter winds. While this means fewer frosts it does encourage more snow for the ski fields and perhaps a feeling that Spring is in the air, when in fact we are actually in the depths of winter.

This would remain in place for 48 hours, and the drift from the area was east to north east, the spokesperson said.

"We don't anticipate any environmental issues as a result of the incident - but any pork found on the shore should not be consumed."

Greater Wellington Regional Council staff had investigated to check whether a pollution response was warranted, but there had been no associated spillage, a spokesperson said.

While it was rare for vehicles to go overboard from vessels - two cars were reportedly lost from the former interisland ferry Aratiki in the mid-1990s - there had been a number of serious incidents involving large vessels plying the waters between the North and South Islands over recent years.

In January this year, the 180m-long cruise liner Azamara Quest sailed perilously close to rocks as it entered the Tory Channel, with more than 1000 people on board.

Since 2011, the interislander ferry, Kaitaki, had been involved in six "near misses", including two cases where other large vessels - one of them an LPG carrier - failed to give way.

In August 2012, the cargo ship AAL Brisbane was headed for Pencarrow Rock in the Wellington Harbour before the pilot came aboard and corrected the course, which had been plotted by a cadet.