Aerial footage showed the whale swimming south of Sydney Harbour trailing a rope attached to a floating buoy.

"It makes it more difficult for the whale to dive," said Pip Jacobs from whale rescue group ORRCA.

"It's tiring for the whale, which is already in a state of distress being tangled."

The whale was about eight metres (25 feet) long, Jacobs said, indicating it was still "quite young".

The rope appeared to be tangled around the whale's left pectoral fin, she said.

"The way it is moving is quite erratic," Jacobs told AFP.

"It's moving south which is unusual.

"They should be heading north as part of their migration."

Teams of volunteers and wildlife rescue experts were searching the coastline to pinpoint the whale's location, she said.

But efforts had been hindered by choppy waters and blustery winds.

"If conditions allow and we have eyes on the whale, the best-case scenario is we have a successful disentanglement.

"If they are dragging gear it hinders their ability to swim freely. The worst-case scenario is the whale can't feed or swim."