Eight-metre dead whale washes onto rocks in Batemans Bay, Australia

© Haberlah family

David and Siwi Haberlah watch as the whale is washed ashore near the Clyde River.

An eight-metre dead whale has been washed onto rocks at the mouth of the Clyde River in Batemans Bay.

Canberra visitor David Haberlah saw the whale on Saturday afternoon, just minutes before waves dumped the mammal in an area known as North Head beach, near Yellow Rock.

With his four-year-old daughter, Siwa, the geologist hurried to the scene.

"I was sitting at the far end and I saw something white floating," Mr Haberlah said. "I thought 'wow, this looks like a massive balloon'. I had a closer look and saw one of the fins come up and realised it was a whale.

"I went running back and Siwa and I went straight to look. It was belly up. It was already dead."

Whale scientist Geoff Ross warned swimmers to avoid the North Head beach because bull sharks and great white sharks would be drawn to the young humpback's carcass.

On Monday, a ship leaves Sydney on a five-day scientific trip to Eden, counting whales returning to Antarctica.

The dead whale is a casualty of that southern migration after the breeding season in warmer northern waters.

Mr Haberlah said the whale was partly submerged but he did notice any injuries.

However, within minutes of striking the rocks, the carcass was gashed.

"It did not look like it had any injuries before but now it is breaking apart," he said.

"You could see within the first 10 to15 minutes that a lot of fresh wounds were opening up as the whale was being washed against the rocks.

"We were not sure how old it was but it measured about eight metres."

By Sunday, the carcass was bloated and floating more clearly above the water line.

Mr Haberlah said he thought he saw a small shark approaching "very fast" at one point.