A rare 250-kilogram turtle found dead on the Shellharbour shoreline had ingested a plastic bag and been hit by a boat, an autopsy has revealed.
The mighty female leatherback sea turtle is thought to have mistaken the bag for a jellyfish.
One of a species that is known to dive more than 1000 metres and spend its lifetime in the open ocean, the creature came ashore to die on Wednesday last week.
Shellharbour City Council brought in heavy lifting equipment to remove the dead turtle from rocks between Shellharbour North Beach and Cowrie Island.
A Taronga Zoo veterinary pathologist performed the autopsy on Thursday but has not pinpointed a definitive cause of death.
The National Parks and Wildlife Service has since buried the creature at a secret whale cemetery.
Its skeleton would be exhumed a year from now and placed on display at the Australian Museum, NPWS senior wildlife management officer Geoffrey Ross said.
"[Turtles] of this size don't come into their hands easily, so scientifically it's a very, very important specimen," he said.
Mr Ross believes the plastic bag, once ingested, limited the creature's ability to dive and led to it being hit by a boat.
"A blockage ... can lead to a build-up of gas in the gut and can impede their ability to dive," he said.
"One plastic bag in the gut can lead to several disorders - they lose condition, they can't eat ..."
Unanderra naturalist Lindsay Smith, co-founder of the Southern Oceans Seabird Study Association, said the creature's demise served as a cautionary tale for those who released plastic into the environment.
"I get very frustrated, particularly when I see people releasing balloons," he said.
"It doesn't matter how biodegradable they are; they still block up seabirds and turtles."
Last Wednesday's recovery was aided by Shellharbour lifeguards and volunteers from Australian Seabird Rescue's South Coast branch, Southern Ocean Seabird Study Association and Australian Seabird Rescue.