Interisland ferry Kaitaki was carrying Graham Hutcheon to Picton, but he was found dead in Wellington Harbour.

The case of a missing ferry passenger has prompted a call for the Interislander to beef up security on its ships.

Maritime New Zealand has disclosed that, in June last year, 69 people boarded Cook Strait ferry Kaitaki in Wellington one Sunday night, but only 68 got off in Picton.

Yet no questions appear to have been asked as to why.

It was only when a Wellington tug spotted a body in the water the next morning that it was realised the sailing had lost a passenger into Wellington Harbour.

The body was that of Graham Hutcheon, 57, of Upper Hutt, who was found dead in the water near Pencarrow.

Documents disclosed under the Official Information Act show police investigated the case and provided assistance to the coroner, who was looking into the cause of death. Maritime NZ did not formally investigate .

Now Hutcheon's widow, Lise, with whom he had two children, is calling on Interislander to beef up security to prevent further similar cases.

She understood ferry crew believed he had left Picton in a car, and were not concerned about a missing passenger.

Had it not been for good weather at the time, the dots might never have been joined, she said. "We were lucky that it was calm and Graham's body was found."

She believed Interislander should have a more robust way of counting the numbers getting on and off ferries. There should also be CCTV cameras installed outside on decks, she said.

"There are CCTV cameras everywhere, so it seems logical to me that they should also be in the common areas outside on ships. This would also help the police should there ever be investigations about anything happening on the ships."

An Interislander spokeswoman said the incident was tragic but, with the case before police and the coroner, she could not comment further.

"The welfare of our passengers is our highest priority and our masters and crew take all practical steps to ensure they are safe when on-board our ships and at our terminals."

Maritime New Zealand was asked for information about deaths or serious injury on New Zealand passenger ships in 2014. Its response included the case of Auckland man Bruce Porter, 56, who was on charter boat Pacific Hideaway in Northland. He hit his head on a propeller as he surfaced and died.

A sailor on Interislander's Stena Alegra got fingers trapped while guiding wires onto a wheel on a pulley block.

Interislander made modifications after the incident to prevent repeat occurrences. The crew member suffered no longterm injuries.

On another ship, Mana II, a crew member got fingers trapped between a rope and bollard and lost the finger ends.

A passenger on Pride of Milford slipped and struck their head coming down stairs when the ship rolled.

The Maritime NZ information also reveals two Interislander ferries are flagged in the United Kingdom.

Its other ferriers , and all of Bluebridge's , were flagged in New Zealand, Maritime NZ advisor Andrew de Montalk said.

Foreign-flagged ships would not normally be covered by New Zealand's Health and Safety in Employment Act, but the two ferries were covered by special circumstances because they were carrying New Zealand passengers and cargo, and operating in New Zealand waters.

Maritime law expert John Knight, of Chapman Tripp, said there was nothing untoward in the ships being flagged in the UK.