A Bluebridge ferry is sailing again less than 24 hours after a trailer crammed with chilled pork broke loose and crashed into the sea during a rough Cook Strait crossing.

The Bluebridge's Straitsman ferry was cancelled yesterday afternoon after a trailer slammed though the railings and toppled into the sea at about 10.30am yesterday.

It was secured with storm-strength heavy chains before setting off on its journey at 8am from Picton, a Strait Shipping spokeswoman said.

The ferry sailed back to Picton at 2.30am today and is scheduled to run as normal today.

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The spokeswoman said an investigation was now under way into how the trailer had fallen overboard the ferry.

It would take several weeks to complete, she said.

"They are looking at a range of possibilities, from crew procedure to the type of cargo on board," she said.

A Maritime New Zealand spokesman said a coastal navigation warning was in place for another 24 hours for any sightings of the trailer.

"So anyone can be on the lookout for debris that may be floating in the area."

However no one had reported any sightings yet and it was likely the trailer had sunk.

He said if it was floating, it would have drifted in a east to northeast direction.

Maritime New Zealand anyone who found pork on the shore shouldn't eat it.

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

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.

"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.

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

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."