Australian authorities were racing to secure a cargo ship drifting off the Great Barrier Reef, with one expert saying it was "sheer luck" it had not hit a reef near the World Heritage-listed site.
The Hong Kong-flagged bulk carrier ID Integrity broke down north of the Queensland city of Cairns late Friday, sparking alarms that it would smash into the coral dive site Shark Reef.
Simon Meyjes, who heads the century-old marine group Australian Reef Pilots, 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 sight 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 towards the Outer Reef, on the edge of the main reef body.
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 Australian Government's emergency towage vessel Pacific Responder was scheduled to be on scene at about 0500 GMT.
"Once on scene it will provide assistance as required and remain on station until the third tug, the PB Leichhardt, arrives and has connected a tow line," AMSA said in an update.
The ship's owner, Hong Kong-based ID Wallem, said its 186-metre-long 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".
Explore further: NOAA establishes 'tipping points' for sea level rise related flooding