Sharks have been seen devouring a dead whale floating close to rocky coastline at South Broulee near Batemans Bay in NSW, forcing authorities to close five beaches.

The young humpback whale was spotted off the rocks at the popular swimming beach on Wednesday morning, according to Stan Wall from Lifeguard Services Australia.

A 100m exclusion zone was set up around the whale and at one point a crowd of more than 300 people gathered to watch.

However, Mr Wall said, after some time, lifeguards in the area were unable to see any spray or air bubbles coming from the animal and it was presumed dead.

"We think it might have come into collision with a boat or maybe even hurt itself on the rocks that we saw it on this morning," Mr Wall said.

Clear signs the animal was seriously ill: ORRCA

Shona Lorigan, from the Organisation for the Rescue and Research of Cetaceans in Australia (ORRCA), said once the animal died, a group of sharks moved in and began devouring the carcass.

The whale was spotted floating very close to a rocky point at South Broulee beach. ( Supplied: Lifesaving Services Australia )

"The sharks are dangerous and are in a feeding frenzy so it's important that everyone stays away," she said.

"It's very important for their own safety - don't go near a dead animal, particularly one that showed signs of serous illness."

She said the whale was seriously underweight and was infested with large numbers of whale lice.

"There really is very clear evidence that the whale itself was sick before it passed away," she said.

Ms Lorigan said four members of ORRCA went straight to the beach after calls from the public.

"We got a large number of calls from the beach this afternoon, and that immediately activated our rescue team," she said.

"While they were in transit... we were on the phone with members of the public advising them on the situation, and we became aware that the animal had passed away."

Beaches remain closed until further notice

Mr Wall said Shark Bay, North Broulee, South Broulee, North Head and Moruya beaches would remain closed until further notice.

He said lifesavers had met with Eurobodalla Shire Council and would do so again early on Thursday morning to re-assess what, if anything, should be done with the carcass.

"We're hoping with the change in tides it actually might get washed out [to sea]," Mr Wall said.

A Westpac Lifesaver helicopter was deployed on Wednesday afternoon to help ensure the area had been cleared of people, he said.