People on board a whale watching boat have battled to save a 10-metre humpback whale caught in fishing rope off Sydney's Bondi Beach.

Dozens of passengers on the boat looked on as the injured and bleeding animal struggled about two nautical miles off the beach on Tuesday afternoon.

Justin Hawco, from Whale Watching Sydney, said the Ocean Dreaming II vessel discovered the exhausted animal close to the start of the whale watching tour.

"We thought it was sleeping, just hanging out near the surface bobbing up and down as they do and I started snapping some photos," he said.

Very quickly Mr Hawco and the crew realised the whale was badly tangled in a line that was attached to a fish trap on the ocean floor.

"In this guy's case [the line] was through the mouth around one of the pectoral fins and then back to the tail and probably six wraps around the tail and from there straight down to a fish trap," he said.

"We figured if we could just manage to get the weight off the whale and let it move freely it might have a fighting chance."

The crew managed to cut the line that ran between the tail and the mouth which helped, but still did not free the whale.

By that stage, the whale "seemed to be almost out of energy", he said.

"It seemed very calm but to me that suggested it had very little energy left," he said.

"It wasn't flopping around, in fact every time we got up close to it, it turned its tail towards us…almost like it wanted us to get in and get the line off."

People on board the Ocean Dreaming II tried to free the whale fishing rope. ( ABC News )

Passengers on the vessel said it had been "amazing" to get so close to the whale but heartbreaking to see it injured.

"At first I didn't know that the whale was injured and I was excited to see the whale, but when I saw it was hurt, it's just heartbreaking," said one onlooker.

Another tourist from Austria said the whale appeared to be "bloody on top".

Another woman who described herself as a regular whale watcher said she had been concerned that the whale would not make it.

"The crew did a wonderful job, it was just so distressing to see a whale in that situation," she said.

"It was just motionless on top of the water, normally they will go dive and spend periods time under the surface…then come up for a couple of minutes, this [whale] was just on the surface the whole time because it was exhausted."

At one point it appeared that there were several other whales circling the trapped whale.

Authorities said the whale had been cut free from some of the rope, but remained trapped in numerous others and was "extremely distressed".

Witnesses said the whale appeared to be exhausted and distressed. ( Supplied: Whale Watching Sydney )

"There's a mixture of things," Marine Rescue New South Wales' Robert Elvin said.

"One of the lines is attached to a very large fish trap and the other lines, they're not quite sure [what they are], there's just quite a tangle of ropes and lines.

"The whale was extremely distressed and exhausted despite the size of it."

The National Parks and Wildlife Service (NPWS) said it was undertaking a visual assessment of the whale by helicopter and was mobilising a specialist crew.

However it was too unsafe to begin disentangling the whale so close to dark and the team said they would begin their work on Wednesday morning.

NPWS director of greater Sydney, Deon Van Rensburg, couldn't say whether the whale would survive the night, but noted they were "fairly resilient".

"It just depends on how much it's entangled and so we'll get an aircraft out there in the morning, we'll get follow-up boats and we'll get there as soon as we possibly can," he said.

He said the whale had become caught in fishing ropes and lines that were floating around the ocean like rubbish.