UPDATED 3.06PM A trailer containing chilled goods was lost overboard in Cook Strait this morning.

Following the incident the ship continued on to Wellington.

Strait Shipping general manager commercial Ed Menzies said they are 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, and the company has launched an immediate investigation into what happened.

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

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

Passengers have described rough seas and huge waves as the unit went overboard.

A passenger on the Bluebridge Straitsman ferry heading from Picton to Wellington which departed at 8am this morning said the truck went over the side of the ship about 10.30am.

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

"There were a lot of waves. There was a side wave and it made the boat go sideways.

"The next thing I know, people were saying a trailer had fallen into the water."

Ms Hakala was sitting near the kitchen when it happened.

"I could hear a lot of stuff falling down in the kitchen."

Ms Hakala said many passengers were vomiting.

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

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

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."

Lorena Ojeda and her daughter Ada were booked in to take a ferry to Picton, but it was now delayed.

"Our ferry was meant to leave at 1.30pm but it's now leaving at 3.30pm.

"They said it was because of mechanical issues."

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

"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."

When the seas became more wild, making him seasick, he saw the waves had grown much larger.

"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.

"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."

Though it was hard to hear it over the howling wind, he described the sound as "metal smashing through metal".

"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."

The truck wasn't afloat long before it sunk beneath the waves.

"Me and my mate were thinking we should have got our phones out earlier, as soon as it started rocking," he said.

"It's not something that you usually get to witness something like that first-hand."

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

A spokesman said Maritime New Zealand officers are at the ship and assessing what has happened, but they had no further details.