A massive oil leak off Australia's northwest coast poses an "immediate risk" to dozens of marine species, with untold numbers possibly dying and sinking to the Timor Sea floor, a report has said.
Biologist James Watson was commissioned to carry out a government survey of the West Atlas drilling rig, where at least 400 barrels of oil have gushed daily into the ocean since August 21.
He found about 20 dead seabirds and a dead sea snake among 23 species, and said the oil appeared to be drawing certain species into the toxic area, where they were at grave risk.
"The presence of dying birds and dead sea snakes suggest that there is an immediate risk to species utilising the water that has been affected by the oil slick," Watson wrote in his report, which was released late Friday.
"It is possible that species that are dying or dead and lying in oil-affected water may not stay afloat for long periods of time, making it unlikely that we would find large numbers of dead animals," he added.
There was a "significant risk" that a change in conditions could push the oil into deeper waters further west and north, where large numbers of endangered cetaceans lived, he added.
"Moreover, as the breeding seasons ends... more juvenile birds will be feeding throughout the region. As such, we believe that over the upcoming months the oil slick -- if uncontained -- may have an increasing impact on these species," he said.
Watson said the spill appeared to have changed the feeding and breeding behaviour of some animals, and long-term studies were needed to determine how severe the impact was.
"I am amazed at how little Australia cares about this," he told the Sydney Morning Herald newspaper.
"This is a huge oil slick."
The rig's Thai-based operator PTTEP Australasia failed in a fourth attempt to plug the leak, now entering its 10th week, and said it could be months before the operation was successful.
More than 2,000 barrels of oil have been recovered from the ocean surface in clean up efforts to date.
Environment Minister Peter Garrett said he remained very concerned about the long-term impact and was "certainly very anxious to see this spill stopped quickly and safely."
"While acknowledging the difficulty of the task at hand, I share the frustrations that efforts to get this spill stopped have so far not been successful," he said.
"However, I am determined that the environmental impacts of this spill are properly assessed and the long term monitoring plan I have commissioned will help ensure that happens."
The World Wide Fund for Nature warned that wildlife was dying and "hundreds if not thousands of dolphins, seabirds and sea-snakes are being exposed to toxic oil" after surveying the spill area, 250 kilometres (155 miles) offshore.
PTTEP Australasia has agreed to pay for environmental monitoring of the area for at least two years.
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