LAMBELLS LAGOON - Heavily armed police are scouring the swamp where they found a pair of board shorts belonging to an 11-year-old girl believed to have been taken by a crocodile.

Up to 10 officers were yesterday wading through chest-deep water at Lambells Lagoon, in Darwin's rural area, searching for more clothing, a body or the animal, Superintendent Michael Murphy said.

The head and tail of a crocodile splashed in the water moments before the missing girl went under at Black Jungle Swamp about late on Sunday afternoon local time.

Superintendent Michael Murphy said the 11-year-old girl had been sitting with her sister, aged 7, and two other children, aged 10 and 12. All of the children were deeply traumatised, he said.

The police squad's efforts were hampered by trees, weeds, flood water and thick vegetation.

One local man said it was mostly freshies that inhabited the creek. Saltwater crocodiles were known to move further inland to feed or breed at this time of year, he said.

"If they are hungry and they can pick up the scent in the creek, they'll come up ... and if the crocodile has taken the child, it's pretty much guaranteed it will have buried it in the reeds or under a rock."

He said the next concern for police was that dingo packs would pick up the scent of the body and maul it.

If confirmed, it would be the second fatal crocodile attack in northern Australia in five weeks. A 5-year-old boy vanished from a river edge in northeastern Australia on February 8, and his remains were later found in the stomach of a 4.3m crocodile.