Australia's Great Barrier Reef Marine Park Authority on Friday approved the dumping of up to three million cubic metres of dredge waste in park waters in a move blasted by environmentalists.

The decision follows the government giving the green light to a major coal port expansion for India's Adani Group on the reef coast in December, under some of the strictest-ever environmental conditions.

It will see Adani dredge three million cubic metres of material from the seabed to allow freighters to dock at the port in Abbot Point, lifting the facility's capacity by 70 percent to make it one of the world's largest coal ports.

Conservationists warned it could hasten the demise of the World Heritage-listed reef, which is already considered to be in "poor" health, with dredging smothering corals and seagrasses and exposing them to poisons and elevated levels of nutrients.

The reef is already facing pressures from climate change, land-based pollution and crown-of-thorns starfish outbreaks.

"This is a sad day for the reef and anyone who cares about its future," said WWF Great Barrier Reef campaigner Richard Leck.

"The World Heritage Committee will take a dim view of this decision, which is in direct contravention of one of its recommendations."

The reef is facing a World Heritage downgrade from UNESCO this year due to concerns about rampant coastal development proposed in the region, particularly port, gas and coal operations. UNESCO are due to meet in June, when they are expected to discuss the issue.

The Great Barrier Reef Marine Park Authority (GBRMPA) chairman Russell Reichelt said he recognised there was intense community concern and debate about the application by North Queensland Bulk Ports Corporation to dispose of dredge spoil in the park.

But he said allowing the project to proceed would help contain development to existing ports, and the reef itself and seagrass meadows would still be protected.

"This approval is in line with the agency's view that port development along the Great Barrier Reef coastline should be limited to existing ports," he said.

"It's important to note the sea floor of the approved disposal area consists of sand, silt and clay and does not contain coral reefs or seagrass beds."

The GBRMPA, whose board is currently under investigation for its links to the mining industry, added that the strict environmental conditions imposed on the project by the federal government would help protect the reef.

The conditions require that sediment entering the marine park be reduced by 150 percent over the long term -- a "net benefit" to water quality -- and that $81 million be contributed to reef conservation programmes and specific measures observed to protect marine flora and fauna.

WWF Australia has said the material dredged during the port expansion would be enough to fill 150,000 dump trucks that "lined up bumper-to-bumper would stretch from Brisbane to Melbourne", a distance of more than 1,000 kilometres (620 miles).