At present the slick has been contained by special floating booms, while the ship itself is being held steady by two tugs, preventing it from further damaging itself on the reef.

However, officials say the situation remains critical with any bad weather threatening to destabilise the ship causing a potential disaster on the world's largest coral reef.

Emptying the ship's tanks of the heavy fuel oil is essential before efforts can be made to refloat the vessel itself and remove it from the reef.

Salvage workers say it may also be necessary to remove the ship's cargo of 72,000 tons of coal, although that will not be clear until the fuel itself has been offloaded.

Officials at Maritime Safety Queensland said the Shen Neng 1 was not within the preferred shipping channel through the reef, but was on an acknowledged alternate route - although obviously off course - when it ran aground.

Previous reports had suggested the ship was taking an illegal shortcut through the Great Barrier Reef Marine Park.