Authorities are investigating a brutal attack on a large saltwater crocodile found with multiple stab wounds and a metal arrow lodged in its spine.

Key points: The Queensland Department of Environment and Science is investigating an attack on a 3.5-metre crocodile in Innisfail, south of Cairns

The Queensland Department of Environment and Science is investigating an attack on a 3.5-metre crocodile in Innisfail, south of Cairns It was found with a large metal arrow lodged in its back and stab wounds to its throat

It was found with a large metal arrow lodged in its back and stab wounds to its throat The crocodile has been euthanased due to its injuries

A member of the public reported the injured crocodile to the Queensland Department of Environment and Science when it was seen swimming around moored boats in the Johnstone River, near Innisfail, on Friday.

Wildlife officers removed the 3.5-metre reptile from the river and found large areas of dead tissue around its spine, where a barbed arrow had pierced through its skin.

A spokesman for the department said there were also gashes to the crocodile's neck and throat, indicating it had been stabbed multiple times with a sharp object.

"The crocodile was found to be in very poor physical condition," he said.

"Officers were able to safely remove some of the exposed part of the steel arrow, but the rest of its barbed shaft remained embedded against the animal's spine."

The spokesman said a decision had been made to euthanase the crocodile on Monday, due to its injuries.

"The arrow is likely to have caused extensive internal injuries and the animal appeared in great discomfort," he said.

Local wildlife guide Brenton Gangemi said it was a terrible loss.

"It is very disheartening to see a native animal in its own home with a spear hanging out of it," Mr Gangemi said.

"Why do people have to be so cruel?

"If inhumane events keep occurring to the crocodiles, this will lead to devastating outcomes for the Johnstone River."

An investigation is underway into the crocodile's injuries.

"It is an offence under the Nature Conservation Act 1992 to deliberately harm or kill an estuarine crocodile, with a maximum penalty of $30,026," the department spokesman said.

Anyone with information is urged to contact the Department of Environment and Science on 1300 130 372.

The Johnstone River is known crocodile territory and members of the public are asked to report sightings and to be croc-wise.