A 24-year-old man is believed to have been grabbed and killed by a crocodile in Australia.

The missing man was swimming with a friend in the croc-infested Mary River, 100km inland from Darwin, northern Australia.

Local police have mounted a search for the man, and Senior Sergeant Peter Lindfield said the man had been attacked near the Mary River Wilderness Retreat.

"Officers from the Water Police Section and Territory Response Group are currently at the scene of the attack and are searching for the man and the crocodile," Sgt Lindfield said.

Mary River Wilderness Retreat and Caravan Park employee Erin Bayard said the missing man's friend was still in shock.

"Two local boys decided to go swimming. We tell people to stay away from the water, they obviously went against this (and) a man was taken.

"We say to everybody it's full of crocs.

"It's one of the most populated rivers in the (Northern) Territory - every couple of kilometres there is a large croc."

Locals say that one monster crocodile 4.5 metres long has often been spotted in the area.

The last fatal crocodile attack in the Northern Territory was in December 2012, when a nine-year-old boy was seized by a saltwater crocodile near the small town of Dhania.

Adults tried to save the boy but the crocodile dragged the child out to deeper water.

In November last year, a seven-year-old girl was taken by a saltwater crocodile near Maningrida.

Three other people, including two children, have been killed by crocodiles since 2009.

In April, a Frenchman had a lucky escape at Nhulunbuy in northern Australia , when he was bitten on the head by a crocodile but managed to fight the creature off by punching it repeatedly.

"It went straight away to the top of my head and diving under the water he tried to do that spinning thing," said Yoann Galeran.

"It was going so fast - everything happened in less than five seconds and then I fell free. I'm very lucky."

Saltwater crocodiles, which can grow up to 7m long and weigh more than a ton, are a common feature of Australia's tropical north.

They have been protected since the 1970s and their numbers have increased steadily since, along with the number of human attacks.

Watch a close encounter with a crocodile