A threatened owl has been released back into the wild after it was found tangled in rope and hanging from a tree at Engadine, in Sydney's south.

Sagar Dave and his family found the powerful owl late on Monday night and called Sydney Wildlife and New South Wales Fire and Rescue for help.

"We were so pleased that the little one was saved," Mr Dave said.

"We were all concerned, it's a beautiful bird and we didn't want anything else to happen to it."

Mr Dave said he initially thought the owl was dead but when they approached it, it started fluttering its wings.

"We thought it was dead because it was lying down, but [I] think it was clever, it was just trying to pretend," he said.

"We said we must do something about it, we can't just leave it.

The powerful owl is on the New South Wales threatened species list. ( ABC News: Luke Rosen )

"We did freak out a bit at first and we didn't know what to do, but we called Sydney Wildlife and they called the firefighters for help.

"The firies were here in about 10 minutes."

Mr Dave's family named the bird Eve, because it was found days before New Year's Eve.

Engadine firefighter Andrew Moser said it was a pretty simple rescue.

"We cut the rope down and then someone held its wings and someone held its big sharp talons," he said.

"We didn't want to release it [straight away] because we were afraid it had a broken wing."

Mr Moser said the rescue was an unusual one for him.

"Usually it is cats and dogs, occasionally it's possums, very rarely do we get birds like this," he said.

"I've always wanted to see one, because I like birds, but I never thought I would get to rescue one."

Powerful owl population declining in Sydney

Firefighter Andrew Moser said the owl's rescue wasn't too difficult. ( ABC News: Luke Rosen )

Jacqui Marlowe from Sydney Wildlife took the owl to the vet where it was nursed back to health.

"It's a magnificent animal, unique and rare and threatened," Ms Marlowe said.

"It's a juvenile, because it does a juvenile begging call.

"Powerful owls are really unusual animals, they're on the New South Wales threatened species list.

"They exert 1.1 tonnes of pressure on possums from their talons and they can rotate their head through 180 degrees."

Ms Marlowe said in recent years the powerful owl population in Sydney was declining and no-one was sure exactly why.

"It could be development and loss of tree cover, it could be a disease, or it could be that there's less possums, because that's their main prey," she said.

"They're not found anywhere else except for in Australia, on the east coast."

She said because 60 to 80 per cent of young die before reaching adulthood there was a program monitoring the powerful owl population in Sydney's north and south.