SYDNEY — A massive oil and gas leak off Australia’s northwest coast was killing seabirds and threatening thousands of marine animals, conservationists warned Friday.

Oil company PTTEP Australasia is preparing to make a fourth attempt at plugging the leaking Montara wellhead, which has been spewing oil, gas, and condensate into the Timor Sea since Aug. 21.

The spill is reportedly Australia’s worst since offshore drilling began more than 40 years ago.

The World Wide Fund for Nature (WWF) said the area around the West Atlas drilling rig was “teeming” with marine life, and 16 seabirds had died after coming into contact with the toxic slick.

Biologist Gilly Llewellyn led a WWF survey of the area and said there were times when they watched as dolphins surfaced “literally in a sea of oil”.

“Clearly, wildlife is dying and hundreds if not thousands of dolphins, seabirds and sea-snakes are being exposed to toxic oil,” she said.

“It’s a massive contamination event, it’s posing a massive risk to wildlife that is largely confined to sea, that we won’t actually see on our shorelines.”

A resources department official told the Senate this week that up to 2,000 barrels of oil were spilling daily into the ocean, much more than the 300-400 estimated by Thai-based PTTEP.

Llewellyn said the combination of the oil and the dispersant chemicals used to contain the spill would have an impact on fish stocks, bird, and other marine life for generations.

“The intensity, the duration and the magnitude and the effect of this oil spill is going to have a huge footprint on an amazing part of our marine world,” she said.

An official government survey was yet to deliver its findings, and Llewellyn said the remoteness of the spill — 250 kilometres (155 miles) offshore — meant the public wasn’t getting a full picture.