A monster crocodile has claimed the swamp as his territory.

A 5m saltwater crocodile has eaten a bull shark at the Kakadu National Park in the Northern Territory.

A group of tourists watched in shock when they saw the croc eating the shark at the upper flood plains of the south Alligator River on Saturday morning.

The shark already had two bites when a photograph was taken.

Tour guide Dean Cameron believed that the croc was at least 3m long and weighed 60kg.

"It would have been amazing to see (the attack) - very wild footage," he said.

"The croc would've taken the shark at night and then it would've had to take it to the bank to eat it."

Two separate boats with about 45 tourists on board each were touring the river as part of the Yellow Water Cruise about 8.15am when they saw the wild feast.

"Nearly 100 people saw it all up and they were jumping for joy," he said.

"They said this had made their Kakadu trip.

"But it was not the first time a crocodile has been witnessed feasting on a shark. Maxine Rawson-Rodriguez was on a jumping crocodile cruise at Adelaide River - about 100km south of Darwin - when the tourist boat came across a another crocodile eating a shark on March 24."

The Northern Territory News released a report on January 29 about how five fisherman fought off a crocodile to get a prized shark caught off a Territory beach.

Mr Cameron, who has been a tour guide for two years and a park ranger for several more, said he would come across a similar scenario once a year.

He said he was not surprised that there are sharks found in the freshwater river system.

"With the wildlife here you just don't know what you'll get to see," he added.

"That's the beauty of it."