Koorana Crocodile Farm owner John Lever found his 4.7-metre crocodile MJ dead, around seven months after the sick animal stopped eating.

MJ measured an impressive 4.7-metre in length. ( Supplied: Koorana Crocodile Farm )

The crocodile was one of thousands living at the central Queensland crocodile farm near Rockhampton.

MJ had a fight with another croc named Big Joe in summer 2018 and he had deteriorated from there.

Mr Lever said when a crocodile dies at the farm it is opened up so its stomach contents can be examined.

The crocodile farmer said he would often find a wide range of things such as rocks, onion bags, broken bottles, hooks and plastic, but what he found in MJ's stomach was something he had not seen before.

"It's the most unusual thing I've ever found," he said

"All the rest of the things, they're dime a dozen, we find them every time we open up a big croc — but this was different.

"There's no number, it's only the style. It's an old-style orthopaedic plate and it came complete with six stainless steel screws.

"Obviously whatever bone he'd taken in that had had the operation performed on it, had been eaten away by the crocodile's stomach juices and just left the stainless steel plate with the six screws in it."

Medical or veterinary?

An orthopaedic plate and six screws were found in MJ's stomach. ( Supplied: Koorana Crocodile Farm )

Mr Lever has contacted someone in England to find the manufacturer of the orthopaedic plate to help identify it.

"We're trying to track it down to see whether it's one used in the veterinary industry or the medical industry," he said.

Once the results come back from England, Mr Lever said he would offer the plate to police for testing if necessary.

"I'd let them have a look at the plate to see if they wanted to do any forensics on it," he said.

"But I can't see they're going to get anything from this plate if it's been in a crocodile's stomach for six years — that we know of — and probably another six to 20 [years] before that," he said.

"Being subjected to the high-acidic content of a crocodile's stomach, I'm surprised it was in such good order as it was — [it is] good stainless steel that's for sure."

Why did MJ die?

MJ was a large crocodile that would have weighed around 700 kilograms when he was in good health.

Mr Lever purchased the crocodile from a farm in north Queensland six years ago, but prior to that he was originally living in the wild.

After an examination of MJ he was not sure exactly what caused the crocodile's death, but noted there were also rocks in his stomach, which he said was normal for the species.

When MJ died, John Lever opened up his stomach to find rocks and a stainless steel orthopaedic plate inside. ( Supplied: Koorana Crocodile Farm )

"There's always stones [in their stomachs] because they actually swallow those and use them to help with their ballast when they're lying in the water," Mr Lever said.

"They move their stomach contents forward enough to get some balance going on.

Sorry, this audio has expired Police brought in to find out who orthopaedic plate found in croc belonged to

"He had some fairly bad lesions on his lungs, and this is fairly common too [with] large crocodiles, the lungs seem to be their weakest organ.

"He had an infection in his lungs somehow, whether this came from the fight he had I don't know, but certainly he wasn't a healthy specimen when we opened him up."

John Lever says the orthopaedic plate was the "most unusual thing" he has found in a dead crocodile. ( Supplied: Koorana Crocodile Farm )

The unsolved mystery

No staff or pets are missing from Koorana Crocodile Farm, so Mr Lever said the origin of the orthopaedic plate is a mystery.

"We're waiting on the results from overseas but I might dig him up and take one of his leg bones [to measure his age]," he said.

"We were so flat out, having to deal with a dead croc in the middle of a busy day when you've got another 5,000 crocs to feed.

"We just got his skin off him as quickly as we could and then dumped the body — buried it.

"On a rainy day when we get time, we can try and age him but even then that's a long process."

John Lever says they tried to feed MJ mince during the months before he died, but he was sick and would spit his food out. ( Supplied: Koorana Crocodile Farm )

Mr Lever said he hoped the public could help identify the steel plate.

"It's certainly got a lot of interest already since we put it up on Facebook. The comments are coming in thick and fast," he said.