As police continue the search for the body of a Darwin man taken by a crocodile in the Mary River over the weekend, an expert has warned of the dangers of swimming in the area.

The 24-year-old was swimming with a friend during a 30th birthday party beside the river, 100 kilometres from the Northern Territory capital, when a crocodile attacked him on Saturday night.

The area is known to have the highest density of saltwater crocodiles in the world, although it is the first known crocodile-related fatality on the Mary River.

Watch Commander Geoff Bahnert says traumatised partygoers witnessed the man being attacked by the large crocodile, recounting the animal swimming upriver with his body in its jaws.

Police have already shot several crocodiles in an attempt to recover the man's body.

Frida Pettersson, who was camping close to where the attack happened, says it serves as a warning about the dangers of swimming in such areas.

"It was my first time there - obviously I'm never going to forget it, and for my children," she said.

"It's just another wake-up call about water safety and to be aware of the water."

Crocodile expert Dr Grahame Webb agreed, adding that swimming in the Mary River came with an almost 100 per cent chance of being taken by a saltwater crocodile.

"They sort of know they shouldn't swim there, but they let their guard down," he said.

"They're having fun ... and do something they wouldn't normally do, and in that particular location it's a pretty tragic situation."

He says nothing more can be done to warn people of the risks of swimming in crocodile-infested waters.

"Parks and Wildlife put a huge effort now into public education, so because we've got a transient population up here it's really important people learn quickly, and I doubt there's much more you can do," he said.

The manager of the nearby Bark Hutt Inn, Shaun Kelly, says the area is well marked with crocodile warning signs but some people still take their chances.

"They may have gone for a swim in the Mary River or in a waterhole in the past and gotten away with it, and this time someone hasn't," he said.