Dog owners told to stay away as dead dolphin washes up at a Thirroul beach

One woman placed flowers by the dead marine mammal, while a long-time local surfer mentioned to the Mercury there were dozens of them swimming underneath him at Sandon Point days earlier.

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Volunteers from ORRCA (the Organisation for the Rescue and Research of Cetaceans in Australia) were alerted around 8am and were on scene shortly after to collect data on the dead male.

Nicci Kennedy said the data would be sent to the National Parks and Wildlife Service (NPWS) to determine how it died and what species of bottle-nosed dolphin it was.

She said dog-owners should keep their pets away.

"You never know what kind of diseases they might have," she said, noting Wollongong Council would send a team to dispose of the carcass.

At this stage Ms Kennedy said it was too early to determine how the creature died, but there were no major signs of trauma to the body.

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She said scratches visible on the underside of the dolphin were common and attributed them to "teeth marks" from other dolphins, from "play or competing for a female".

Ms Kennedy said there wasn't a lot of specific research into dolphins in the Illawarra, though people could record their sightings by contacting Dolphin Research Australia.

A NPWS spokeswoman said factors that are routinely investigated include screening for infection, diseases and examination for signs of trauma from natural and anthropogenic sources - though this particular animal has not show obvious signs of disease either.

"This may simply be an older animal whose time has come," the NPWS spokeswoman said.

"Bottlenose dolphin are relatively common in NSW coastal water and not unsurprisingly they are also one the most common stranded marine mammal species in NSW, even if infrequently."

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