Curious crowds have flocked to a northern Sydney beach to see a humpback whale that washed up dead overnight.
A spokesman for the NSW National Parks and Wildlife (NPWS) said the whale is believed to have died at sea before being washed into an ocean pool at Newport Beach.
It was estimated to be around 10 metres in length and to weigh between 20 and 25 tonnes, he said. The whale is also believed to be juvenile.
Mums and dads with toddlers in tow joined tourists among the interested spectators.
Steve Messenger, on holidays from Newport Beach in California, was visiting the area to compare the Sydney beach to the one in his hometown.
"It was a little bit gross," Mr Messenger told AAP.
"It was like a big pile of jelly, like a big blob. You can't see the head or the tail."
The ocean pool that the whale washed into, crushing the bollards and chains around it, has been closed off as rangers decide how to remove the dead mammal.
A sign erected by the council also says the beach has been closed, warning of a possible increase in shark activity.
"Whenever you get a dead whale, there's a chance of pieces of dead whale, blubber or oil washed out there, and that will attract sharks," NPWS northern beaches manager Chris Grudnoff said.
"A whale like this would be a very good feed for a pack of sharks."
The situation would be reassessed after low tide, Mr Grudnoff said, but it could take two days to get rid of the whale.
A tugboat or a 20-tonne excavator could be used to remove it, or authorities would wait for the 1.9 metre swell expected on Wednesday night to wash it out of the ocean pool.
"We will be possibly be leaving it up to nature after our assessment at low tide," Mr Grudnoff said.
One of the problems associated with the carcass was the strong and repulsive smell at the beach, he said.
The NSW Environment Department later said on its Twitter feed that the whale is believed to have died in the last two days.
There was no evidence of boat strike or entanglement, it said, although the whale seemed to have suffered some shark bites after its death.