A dolphin rescued off Sydney's northern beaches earlier this week has been euthanased after becoming stranded for a second time.

It took six men to refloat the three-metre Risso's dolphin after they found it in trouble on Curl Curl Beach on Monday.

The mature female dolphin went back out to sea but became stranded again at Kurnell, in Sydney's south, on Tuesday evening.

An autopsy performed at Taronga Zoo revealed a plastic bag was blocking its stomach, Organisation for the Rescue and Research of Cetaceans in Australia (ORRCA) president Ronnie Ling said.

"It also had a massive infection it its chest area and we also noted that the animal was very thin," he said.

"We don't know what was in that plastic bag and, on top of this chest infection as well, I think the numbers were really against this poor animal."

The dolphin would normally live in a pod, and ORRCA checks for mass strandings when finding a single dolphin.

Mr Ling said people who find stranded sea animals should provide them with shade and wait for ORRCA experts to arrive, rather than trying to help them out to sea themselves.

"We seem to get better results when we have that rehabilitation time," he said.

"Because the animal has stranded for a reason, it gives a better insight into the animal and to its condition and chances of rehabilitation and release."

ORRCA's 24-hour emergency hotline can be reached on 02 9415 3333.