The capture of a massive saltwater crocodile - nearly five metres long - in Darwin Harbour and another four of the reptiles caught near populated areas has prompted a Northern Territory mayor to call for a cull.

The Katherine Mayor Fay Miller says crocodiles were increasingly encroaching on built-up areas and action was needed, including a cull.

However NT Parks and Wildlife denies there has been an extra influx.

A 4.7-metre saltwater crocodile was spotted by a fisherman in a trap in the harbour on Monday.

Rangers could not fit the animal inside their boat and it was secured alongside.

Crocodiles of that size were generally never seen around the harbour and rangers believe the croc, who was missing part of his tail, might have been bullied out of his normal river home by a younger rival.

Captured crocs are often destroyed but this creature's large size might lead to him being sent to a tourist spot.

Rangers have also pulled out four large saltwater crocs in the Katherine area this month, near populated areas.

That included at the high level bridge for the first time in two decades and within 2km of the popular Katherine Hot Springs attraction.

"It is time for some more serious steps to be taken, because very large crocodiles are starting to encroach too close to civilised built-up areas," Ms Miller told ABC radio.

"I will certainly have a conversation with my local MP (Sandra Nelson) and see that she actually lobbies government for that (a crocodile cull) as well."

"This is a very big wake-up call, there really doesn't seem to be anywhere that is safe to swim."

However Acting Director of Northern Australian Parks, Lincoln Wilson, said crocodile management had been normal for this time of year.

"High water levels after the wet season make it easy for crocodiles to move around, and you should be crocwise around any body of water in the Top End," he said.

The NT government has been contacted for comment.