An Australian tug boat Sunday reached a cargo ship which had been drifting off the Great Barrier Reef, as environmentalists warned that greater shipping traffic could harm the world's biggest coral reef.

The Hong Kong-flagged bulk carrier ID Integrity broke down north of the Queensland city of Cairns on Friday, sparking alarm that the 186-metre (610-foot), 45,000 tonne bulk carrier would smash into a reef near the World Heritage-listed site.

Simon Meyjes, who heads the century-old marine group Australian Reef Pilots, said it was "sheer luck" that the Integrity had not run aground at the dive site Shark Reef or nearby.

He said it appeared an environmental disaster had been averted by the crew dumping some of the Integrity's sea water ballast so it passed over Shark Reef.

"I don't know what the actual under keel clearance would have been as it went over the reef," he told AFP. "But certainly it would have been a very, very uncomfortable situation. They were at the mercy of the weather completely.

"I think that they have been very, very lucky."

Meyjes said had the Townsville-bound Integrity drifted slightly further to the north, it could have hit Osprey Reef, an exposed coral outcrop.

"So it's just sheer luck," he said. "We're all breathing a huge sigh of relief I can assure you."

The ship, which suffered an engine breakdown en route from Shanghai, was reached by a commercial tug, PT Kotor, on Sunday as it drifted in open water in the Coral Sea.

The Australian Maritime Safety Authority (AMSA) said the tug had connected a towline to the Integrity and they were travelling slowly away from the Outer Reef and awaiting the arrival of the larger tugs.

The ship's owner, Hong Kong-based ID Wallem, said its vessel was empty and there had been no pollution spills, adding it would "take measures to avoid any environmental impact in Australian waters".

But the incident has angered conservationists who have long raised fears about the impact on the Great Barrier Reef, particularly from shipping, from Queensland's coal and gas boom.

The Chinese-registered coal carrier Shen Neng 1 foundered in April 2010 leaking tonnes of heavy fuel oil and threatening an ecological disaster.

A major catastrophe was ultimately avoided but the huge ship gouged a three kilometre-long (1.8 mile) scar in the world's biggest coral reef and was stranded for nine days before salvagers could refloat it.

Environmental group WWF Australia said in the latest incident, a major disaster had been averted by only a matter of metres.

It said the forecast increase in shipping traffic in the waters off Queensland was "a risky game of Russian roulette that is destined to end in disaster".

The independent activist group GetUp! said the stricken Integrity proved it would be reckless to increase traffic in the region.

"The incident should be of concern to all Australians. It's more likely to occur in the future as we see more and more ships use the Great Barrier Reef to export coal," national director Simon Sheikh told ABC Radio.

But Queensland Premier Campbell Newman said the theory that more ships would necessarily result in more accidents near the reef did not stand up.

"If that was the case then people would be involved in far more plane crashes today than we saw 20 years ago, and clearly that's not the case," he said.

Earlier this year representatives from UNESCO visited Queensland to inspect the reef to check for any impact from coal and gas projects, which use ports near the Great Barrier Reef to export their products. They have yet to report.