BRISBANE - The risk of a major oil spill from a coal carrier grounded on the Great Barrier Reef is diminishing, with the majority of its stock unloaded before its refloating planned for this week.

The Chinese-registered Shen Neng 1 was carrying 975 tonnes of fuel oil when it ran into Douglas Shoal in waters off Rockhampton.

The oil from the ship is being transferred off to avoid the risk of a spill during salvage operations. The transfer began on Friday morning and is expected to finish today, Maritime Safety Queensland said.

"If current conditions hold, we anticipate finishing the operation by [today], however safety, not speed, is our focus," general manager Patrick Quirk said.

"The careful transfer of oil and the safety of our crews is our main priority and our goal continues to be no oil on the water."

The ship has leaked three to four tonnes of heavy fuel oil on to the reef, but it was mainly lost on the day it ran aground.

Quirk said a small amount of oily water had escaped yesterday from the ship's ballast tank, which is used to help the vessel move more efficiently by lowering its centre of gravity.