A birthday celebration in the Top End has ended in tragedy after a man was attacked by a crocodile and is believed dead.
Police say a 26-year-old Darwin man was celebrating a friend's 30th birthday on Saturday at the Mary River Wilderness Retreat, about halfway between Darwin and Kakadu National Park.
At around 3pm (CST), two male partygoers were swimming across the muddy river when one was grabbed by a five-metre long saltwater crocodile.
"The Mary River is known worldwide to have the greatest saturation of adult saltwater crocodiles in the world. You don't swim in the Mary River," watch commander Geoff Bahnert told news agency AAP.
He said alcohol may have played a part in the decision to take a dip.
"Several of the group in the party witnessed the male being taken in the jaws of the croc for a period of time, and then he was out of sight," he said.
Police and the NT Crocodile Management Team arrived at the lodge and caravan park before last light on Saturday, and an officer shot one of the largest crocs found in the area.
It's not yet known whether it was the attacking crocodile.
"That was to make sure that it wouldn't hamper any rescue attempts - large crocodiles are territorial," Sen Sgt Bahnert said.
The victim's parents and sister drove to the site from Katherine to be briefed by police on what is now most likely a recovery operation.
Police are still taking statements from traumatised witnesses, and dragging the river in search of the body.
"The Mary River is a tidal river, with lots of water movement," Sen Sgt Bahnert said.
"It's 20 hours since he's been missing and he hasn't been located yet.
"We hold the gravest of fears."
The advice to tourists is to come, look, take photographs and stay out of the water.
"We advise people check the water before entering swimming pools - that's how seriously we take water safety in relation to crocodiles," Sen Sgt Bahnert said.
"Any body of water you're going into in the NT, there's the potential for a croc to be in there."