Bodies could be recovered - salvage expert

New Zealand has the technology to recover corpses from the Tongan ferry Princess Ashika, says a salvage expert.

Keith Gordon said remote-operated vehicles (ROV) were available but the question was who footed the bill.

"If it was rich Americans, they (relatives) would go out and finance it," he said.

Mr Gordon - who has his own remote operated vehicles - is best known for his book Deep Water Gold on the salvage of a wartime wreck, the RMS Niagara, off the Hen and Chicken Islands.

New Zealand and Australian salvage workers used makeshift equipment to recover 555 gold bars from a strong room deep in the hull of the ocean liner in 1941 - at a depth of 121 metres.

They used a human observer in a diving chamber, watching through glass portholes to direct the use of claw grabs by an operator on the surface, but Mr Gordon said that kind of real-time observation could now be done with ROVs.

The Princess Ashika is sitting on the sea floor at a depth of 110m.

It sank with the loss of 75 lives of which 73 bodies are still missing.

One possibility would be for the passenger lounge to be torn open so that the bodies could float to the surface.

"If they are in the upper structure, that could be relatively easy to open up," Mr Gordon said. "You could just tear the top part open ... the bodies would just float up".

"No doubt, a ROV could even recover a number of bodies or assist to do that," he said. "You have an articulator arm on the front - we've used them for that type of thing before, to bring a body up".

Some overseas police forces, such as in Scotland, actually had an ROV specifically for recovering bodies.

"For smaller ROVs these days, that depth is no big deal".

The larger of two ROVs used by his company, SeaRov Technologies, can operate to 330m depth. Mr Gordon said that it was possible that the Tongan Government might reduce costs by simply declaring the wreck as a sea grave, and block efforts by families to cut loose their dead relations.

He thought it unlikely anyone would pay to salvage the ship, even though that might give definitive evidence of its seaworthiness before the sinking.

New Zealand Diving and Salvage managing director Dougal Fergus has said that for human divers to work deeper than 50m would require use of an oxygen, helium and hydrogen mix and compression and decompression chambers.

Divers must spend at least two weeks in the chambers before the dive and the operation could take at least a month.

Adding the cost of a ship and crew would mean a bill of between $750,000 and $850,000 a day, putting the total bill up toward $25 million.

NZ Navy personnel on the dive ship HMNZS Manawanui today released graphic pictures of the ferry sitting upright on the seafloor.

There was no sign in those photos of the 73 people still missing, but thought to have been on the ferry when it sank around midnight on August 5, 86km northeast of the island's capital of Nuku'alofa.