Cassius, who was captured in the Finniss River near Darwin in the Northern Territory in 1984, died suddenly on Friday at the Marineland Melanesia Crocodile Habitat on Green Island, off Cairns.

The saltie, who stretched to 5.5m – or the length of a Ford Ranger – had not eaten for a fortnight, although that was not unusual behaviour.

Professor Sally Isberg, from the Centre Crocodile Research, gave Cassius a health check on October 14 and all seemed well.

But days later he began declining food.

The keepers found him dead in his enclosure about 11am, at the age of roughly 121.

As Cassius was born in the wild, there was no accurate way to determine his true age, but Scott said he “lived well beyond his natural years … having his own space and not being challenged”.

Cassius was plucked from the wild not long after a series of crocodile attacks on boats in the NT.

Eventually, another crocodile called Sweetheart was found to be the culprit and killed.

But Cassius, Scott said, turned out to be the real “sweetheart”.

In 1987, George Craig, an adventurer, hunter and founder of the Cairns facility – and Scott’s grandfather – relocated Cassius to Green Island and an almost four-decade friendship began.

“They did have a close bond,” Scott said.

“George would quietly sit with Cassius for hours on end.

“In the last couple of years, George has been getting around on a mobility scooter and every time he came near the enclosure, Cassius would come over to him.”

Craig, 94, moved to an assisted care residence in early October.

Asked if Cassius had felt lonely without his mate, Scott said there might be “some kind of correlation”.

“Cassius was very aware of George and his presence,” he told AAP.

Scott said it was hard to tell his grandfather that Cassius had died – although, while it was a surprise, it’s not uncommon for a crocodile to be sick for months.

“They’re just really good at surviving,” he said. “But George knows that time catches up with everyone.”

In 2011, Cassius took out the Guinness World Record for the largest crocodile in captivity, measuring 5.48m in length and weighing more than a tonne.

He briefly lost the title to Lolong, a saltwater crocodile in the Philippines, who clocked in at 6.17m.

Lolong died in 2013 after contracting pneumonia.

While the cause of Cassius’ death is unknown, there are no immediate concerns about what might have caused it.

Scientists will visit the facility to examine Cassius’ body, which Scott said offered “unprecedented data”.