A massive saltwater crocodile - said to be one of the biggest ever seen in Queensland - has been shot dead in Rockhampton.

Police and state environmental officers are investigating after the 5.2m male reptile was found with a bullet in its head in the Fitzroy River on Thursday.

The monster crocodile was taken to the nearby Koorana Crocodile Farm, where it will be buried once a necropsy is carried out.

Farm owner John Leaver said a 5m-long crocodile had not been caught in Queensland for 20 to 30 years.

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"There may have been some others shot in the wild that we don't know about, but from my recollection, over the past three decades this would be the largest," he told AAP on Friday.

Leaver, who ran a crocodile removal service across the state for 20 years, said the largest one he ever caught was 4.95m in the late 1980s.

"We caught that one up near Airlie Beach," he said.

Leaver said it was not unusual for a crocodile of that size to be found so far south.

The farmer said Rockhampton locals used to shoot crocodiles "equal to that size" in the 1930s, 40s and 50s, before it was illegal.

It is believed the 5.2m reptile had been dead for a few days before a member of the public spotted it floating and notified environmental officers.

Leaver said the gunshot had caused a large hole in the top of the saltie's skull, suggesting the bullet came from a "fairly large calibre rifle".

"I would say that someone felt very threatened," he said, when asked why he thought someone shot it.

Cassius, a male caught in the Northern Territory three decades ago, is recognised as the world's largest crocodile in captivity.

The aquatic reptile measures 5.48m and lives at a farm on Green Island in far north Queensland.

A 6.16m Philippino crocodile called Lolong previously held the record before it died in 2013.

Mayor Cox Elorde of Bunawan township handles the corpse of Lolong, the record-holder for largest crocodile in captivity, at the conservation park in 2013 in Bunawan, Philippines. Photo / Getty Images
Mayor Cox Elorde of Bunawan township handles the corpse of Lolong, the record-holder for largest crocodile in captivity, at the conservation park in 2013 in Bunawan, Philippines. Photo / Getty Images

Meanwhile, a wildlife expert says the death of the crocodile would likely create a void in a central Queensland river that could result in younger males becoming more aggressive as they battle for dominance.

Environment southern wildlife operations director Michael Joyce said the remaining male crocodiles could act differently and become more hostile as they establish who will rule next.

"They don't necessarily become more aggressive with outsiders," Joyce told AAP on Friday.

"But we would expect people to be croc-wise in croc country and be extra vigilant."

Joyce says wildlife officers will monitor the river to see what transpires and who becomes the next dominant male.

"The whole thing could be over in 24 hours; at other times it could take months to see a slight move in the population," he said.

CROCO-METRE - SOME OF THE WORLD'S LARGEST CROCODILES

  • Cassius, the 5.48m giant holds the Guinness World Record for being the largest croc currently in captivity. Housed on Green Island in far north Queensland, he weighs more than one tonne and is believed to be more than 110 years old.
  • The world record was previously held by Lolong in the Philippines. Lolong measured a massive 6.17m before died in 2013.
  • Gustave, who resides in the Ruzizi River in Burundi, is thought to be the biggest crocodile currently alive. He's estimated to be 6m long and is rumoured to have killed more than 100 humans in his time near Lake Tanganyika.
  • Polish immigrant Krystina Pawlowski claims she shot an 8.6m crocodile on the Norman River near Queensland's Gulf of Carpentaria in the 1950s. It's Australia's largest known crocodile but its size has never been verified.
  • Australian outback wrangler Matt Wright is thought to have caught the country's second-largest crocodile in the Northern Territory in late 2015. It measured 5.58m.
  • Well-known Northern Territory reptile Brutus, located in the Adelaide River, is estimated to be about 5.5m in length.
  • Brutus has an even bigger nearby neighbour. Dominator has never been officially measured but it's estimated he measures up to 6.1m.
  • A 5.2m saltie was found floating in the Fitzroy River near Rockhampton on Thursday with a gunshot wound to its head. Local croc farm owner John Leaver says a beast that size has not been caught in Queensland for 30 years.