Emergency crews and croc catchers are searching a Top End river where a Darwin man was taken by a crocodile while swimming.

The 24-year-old man was swimming with a friend yesterday afternoon near the Mary River Wilderness Retreat, about 110 kilometres east of Darwin, when a crocodile attacked.

Locals say a 4.5-metre crocodile is often seen in the same area.

The man's family are travelling to the site at the Mary River.

Staff at the holiday park say the man was warned not to swim in the water.

The wilderness retreat's Erin Bayard says crocodiles are very common in the Mary River.

"We do actually tell all our guests not to swim and to stay at least five metres back from the edge of the bank," she said.

Croc catchers have arrived at the site and begun searching for the man.

One witness says he heard up to 20 gunshots in the area where police officers were searching last night.