The creature - a rare leatherback sea turtle - was discovered on Wednesday morning on rocks between Shellharbour North Beach and Cowrie Island.

Multiple people had reported seeing the creature swimming a day earlier, unable to dive and showing signs of "distress", however the National Parks and Wildlife Service could not find it.

The turtle has been collected by NPWS personnel, and will undergo an autopsy and likely end up in the Australian Museum.

Unanderra naturalist Lindsay Smith said it was possible the turtle died as a result of swallowing plastic, but he favoured the theory it died of old age.

"The animal had a couple of holes in its flippers, which to me indicated that it was a tagged animal at some stage," said Mr Smith, president of the Southern Oceans Seabird Study Association Inc.

"Tags don't come out easily; they take a long time.

"It also had cataracts or something over both eyes - that was another thing that made me think it was very old."

Mr Smith estimated the animal was 1.5 metres long, with flippers spanning two metres, making it a small example of its species.

Unlike other turtles, it had no hard plates on its back, only skin and oily flesh. It was, said Mr Smith, "one of the most incredible animals I've ever seen".

"We know so little about these creatures because the only time they come ashore is to lay eggs or because they're sick or old," he said.

"There were some children at the rocks [where the turtle died]. I said, enjoy it, because you may not get to see another one in your lifetime."

Wednesday's recovery effort was aided by volunteers from Australian Seabird Rescue's South Coast branch.

Representatives from Southern Ocean Seabird Study Association, Australian Seabird Rescue and Shellharbour lifeguards were also involved in the rescue.