Royal New Zealand Navy divers on the scene above HMNZS Manawanui, off the southern coast of Upulo. Photo: Supplied / New Zealand Defence Force
A salvage expert says he expects it could take up to $40 million to clean up the Manawanui wreck.
Up to 200,000 litres of fuel has leaked into the sea when the ship sank off the southern coast of Upolu in Samoa just over a week ago.
It has 950 tonnes in main tanks which are still believed to be intact.
The navy says the salvaging of HMNZS Manawanui cannot be rushed and it is a complex operation that will take some time.
Rena salvage operation diver speaks on Manawanui wreck
Deep Dive Division founder and New Zealand Samoan Tua Karalus worked on the Rena salvage operation for two years.
He said it would be very tough logistically to salvage Manawanui.
The ship is in an isolated area off the southern coast of Upulo and reported to be about 30 metres down, Karalus said.
"Which is okay for salvaging vessels because it means that people can actually dive down and do what they need to do on it. Anything deeper than 50 metres makes things a bit more complicated where we have to breathe mixed gas and get a dive saturation team in, you know using dive bells and all sorts of things like that."
Karalus said they were able to successfully pump thousands of litres of crude oil from the Rena after she sank.
But there were reports that the Manawanui had 900,000 litres of oil on board and that 200,000 litres of oil had already leaked.
"It means that they'll have to (a) find the leak, (b) shut it off with different valves etcetera and then (c) they're going to have to do a technique called hot tapping where you're putting specialised valves into the tank there, the fuel tank, and it basically cuts into the hole but securing that fuel inside and then it's able to be pumped out from depth into storage containers."
It was a technique that had been used overseas, he said.
One example was the Manolis L cargo ship which sank off the Canadian coast in 1985.
All of the 400,000 litres of fuel on board was finally removed in 2018 and it was at a depth of 70 metres, he said.
"They used ROVs [remotely operated vehicles], they're like robots, to hot tap into it and secure that fuel. It's something that absolutely can be done."
Another risk factor for causing damage to the main fuel tanks might include weather, for example, a cyclone might move the wreck, he said.
It would be a difficult salvage operation, but it would be possible, he said.
"It can be done, things can be done if we do throw enough money at it ... you know to salvage the Manolis L cost US$23 million, so $40 million you could probably do something really good to secure the environment there."