Six Greenpeace activists boarded a coal ship bound for South Korea near Australia's Great Barrier Reef on Wednesday, protesting against the expansion of the rich Australian coal industry and its impact on the World Heritage site.

Environmentalists say the Great Barrier Reef, a popular tourist site worth about A$6 billion ($6.1 billion) a year to the Australia economy, is threatened by dredging, sedimentation and coal port and shipping development.

UNESCO will decide in June whether the reef should be listed as a World Heritage Site in danger.

The ship MV Meister was carrying thermal coal from Abbot Point in northern Queensland state, a port that falls within the Great Barrier Reef heritage area, and was still in Australian waters in the Coral Sea when it was boarded en route to Donghae in South Korea.

Activists launched inflatable boats from the Rainbow Warrior and boarded the coal vessel early on Wednesday.

"Please be advised Sir that we have six volunteers boarding your vessel," Peter Willcox, Captain of the Greenpeace flagship Rainbow Warrior, told the MV Meister by radio.

"They will not interfere with your ship or its equipment and they will remain on deck," he said.

"We've established a peaceful protest on board this ship because the Australian government has utterly failed to do anything to reduce our biggest single contribution to climate change which is our coal export industry," said Georgina Woods, a climate campaigner onboard the Rainbow Warrior.

A letter was handed to the captain of the ship detailing their reasons for the occupation.

The Australian Coal Association, an advocacy body representing the industry, said the action was dangerous and irresponsible.

Coal is Australia's second-largest commodity, with exports increasing some 50 percent over the past 10 years and worth almost A$60 billion in 2011-2012.