A young humpback whale is "swimming freely" after a desperate rescue operation successfully released the mammal after nearly 24 hours tangled in ropes in Sydney Harbour.
The difficult mission started on Thursday and ended late on Friday morning when the distressed whale was cut away from the ropes that had wrapped around its tail.
The humpback, which was estimated to be between 10m and 15m long, swam straight towards the Sydney Heads and into the open ocean.
NSW National Parks and Wildlife Service area manager Ben Khan said the mammal was "swimming freely and disentangled" after whale-release specialists managed to sever the ropes.
It continued to be watched by marine experts as it moved out of the harbour, he told reporters.
Jessica Fox, from volunteer rescue group ORRCA, said it was common for whales to enter Sydney Harbour, but it was very rare to see one of them become entangled.
She told AAP it was not known how the whale became caught up, but it happened before it made its way into the harbour.
The "wonderful" release came after buoys were used to help keep the animal relatively still as specialised staff worked to get it free.
As the whale swam between Bradley's Head and the Manly headlands, wildlife officers used a long pole tipped with specialised blades to remove the ropes.
Members of the public first noticed the stricken whale about 1pm on Thursday and reported it to ORRCA's hotline.
Mr Khan said the service's whale disentanglement team started its operation in the afternoon but were defeated by sunset.
"It's obviously a very complicated operation. We've got a live animal that's very, very large," he said.
Authorities praised commercial vessels operating on Sydney Harbour near the whale, saying they had abided by exclusion zones that allowed rescuers to free the entangled whale.
Humpback whales sail north for warmer waters in June and July, then return south in September and November.
The peak time for humpback whales to pass Sydney Harbour on their way south is in late September, according to ORRCA.
Humpbacks are the main species sighted off NSW, with their numbers on the rise due to conservation efforts.
