A trailer unit went overboard during a rough sailing of the Bluebridge's Straitsman ferry.

Swinging pig carcasses are likely to have led to a truck trailer spilling overboard from a Cook Strait ferry.

A B-train trailer carrying two shipping containers filled with chilled pork, and the tug unit attached to it, broke free of their chains, smashed through a railing and toppled off Bluebridge's Straitsman ferry on July 18.

The refrigerated containers, which held chilled pork, tumbled into the sea, along with a ship's tug unit, worth about $300,000, that shifts containers about.

KEVIN STENT/FAIRFAX NZ Bluebridge ferry passenger talks about the shock of witnessing a trailer unit going overboard during a wild Cook Strait crossing.

Maritime NZ said on Wednesday it had completed its initial inquires into the incident.

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A spokesman said it was not possible to determine the exact reason for the loss of cargo, but "the suspended pork carcasses are likely to have increased the forces on the tug and B-train trailer lashings in the heavy seas".

JORDAN MATCHITT The trailer unit after it went overboard from the Straitsman in July.

Maritime NZ was satisfied the ferry's owner, Strait Shipping, was taking appropriate actions in response to the incident, he said.

"Maritime NZ is working with Strait Shipping to review their lashing and surveillance guidelines and identify any lessons for all parties, including the transport companies who contracted Strait Shipping to take the pork contained in the B-train trailers to the North Island," the spokesman said.

Strait Shipping commercial manager Ed Menzies said the company had investigated the incident and was also "unable to determine exactly what caused the cargo to shift".

FAIRFAX NZ A truck rolled off the Bluebridge ferry during a wild Cook Strait crossing.

"We have reviewed our practices, and those of our freight customers, and we are confident that this was an isolated event, unlikely to occur again," Menzies said.

The containers and trailer belonged to Freight company TNL.

There has been no sign of the containers, the tug, or the pork, since the accident.