A claim by a senior salvage advisor in the first book on the Rena disaster released today is being refuted by Svitzer salvors working on the wreck, and Maritime New Zealand.

Quoted in the book Black Tide written by John Julian, Captain Ian Tew says authorities may have missed a chance to refloat the container ship in the days after it hit the Astrolabe Reef on October 5, 2011.

The Rena when it first struck Astrolabe Reef on October 5.

The experienced salvage master claims the Rena could have been removed from the Astrolabe Reef using tugs, its main engine and heavy swells that pushed the ship onto a sharp list in stormy weather.

"The heavy swell might well have reduced the ground reaction to nothing and the rock she was impaled on might have broken off."

In a joint written response, Maritime NZ, London Offshore Consultants, and Svitzer Salvage says no tug in the world is strong enough to have pulled the ship free.

The response says even all the tugs physically available in the area would not have had sufficient pull to move the ship an inch.

'Even had it been physically possible, trying to rip the Rena off the reef quickly with tugs when the vessel was so badly damaged would likely have resulted in much more fuel oil and many more containers spilling into the sea.

'It also would have created enormous and unnecessary safety risks.”

The ship's bottom was destroyed when the ship hit the reef and tugs would have had to overcome 11,000 tonnes of ground pressure.

The Port of Tauranga's newest tug Sir Robert has a bollard pull of 50 tonnes. The salvor's vessel Go Canopus which arrived on scene after the wreck, has a bollard pull of 149 tonnes. The tug Singapore which towed the barge Smit Borneo to Tauranga, has a bollard pull of 189 tonnes.

The joint statement reports salvage experts and MNZ salvage experts boarded Rena the same day the vessel grounded and began the assessment of the Rena's status.

The assessment found Rena was firmly grounded and had suffered substantial structural damage resulting in leaking fuel due to the high speed in which it hit the reef.

Most of the underwater forward spaces were breached and flooded, including two cargo holds, with water also entering into a third cargo hold.

The duct keel which runs the length of the bottom of the ship containing fuel lines and pipework was also believed to be breached and leaking oil.

The report says given the vessel's fragile condition it was neither prudent nor safe to attempt to pull the vessel off the reef. That assessment of the ship's physical condition is the key aspect of any salvage operation.

In the case of the Rena, the team of experts spent the next few days completing a very thorough assessment of the ship and the damage sustained in the grounding.

Following the assessment MNZ says specialised salvage equipment was sought from New Zealand and abroad, Svitzer salvors were mobilised and MNZ's appointed salvage advisors and naval architects - London Offshore Consultants (LOC) arrived on board Rena the following day.

Both Svitzer and LOC also drew on their international teams of naval architects and engineers, who worked round the clock undertaking reviews and calculations to ensure the accuracy of the on-board assessments.

Those involved comprise some of the best salvage experts, naval architects and maritime experts in the world.