A Darwin fisherman rescued a snagged turtle after he found a one-tonne ghost net drifting off the Northern Territory coast.

Ian Johnson encountered the net while on a week-long fishing trip off the Cobourg Peninsula, which divides the Van Diemen Gulf and the Arafura Sea about 350 kilometres east of Darwin.

"We could see turtles flapping in it, chunks of fish in there," he said.

"We could smell a bit of death in there," Ian Johnson said about the ghost net drying on land. ( Supplied: Ian Johnson )

"Eventually there was a big shark come in to get a feed of broken-up bits of barracuda and what not."

"It was huge, it was so big."

The ranger at the nearby Black Point Ranger Station, Alan Withers, told 105.7 ABC Darwin it was the largest ghost net, a term given to nets abandoned at sea, he had ever seen.

"She would have punched close to a tonne," he said.

"We needed three vessels to drag it off. We enlisted the aid of a customs vessel, the Roebuck Bay.

"We ran a rope back to a shackle and made it like a giant purse string.

"Back on the boat ramp it took a tractor in low four and a Toyota chained to the front to move it up the boat ramp."

The ghost net has been left to dry on the boat ramp before it will be burned.

'We could smell a bit of death in there'

Mr Johnson said he and his wife Elshonner and their friend Brooke Summers had seen dead turtles caught in the net.

"We noticed a lot of buoys on top of the water bunched in a group," Mr Johnson said.

"So we went over and had a quick flick at it. Realised it was a ghost net.

"You could just see - because the water was so clear - you could see it going straight down towards the bottom."

He said the ghost net was at least 10 metres deep.

"It looked like a twister, so to speak. It went all the way down," he said.

"We cut out a little turtle that was in the top of it and released him.

"For the next three days we were fishing out there it was sitting at the boat ramp. It was drying up there.

"We could smell a bit of death in there as well.

"There were a lot of little crabs, and some turtle shells. There were three or four other turtles in there. It must have been floating for a while."