A giant north Queensland crocodile has been crowned by Guinness World Records as the biggest crocodile in captivity.

Cassius, named after legendary boxer Cassius Clay who later became Muhammad Ali, was caught in the Northern Territory in 1984, but has called Green Island, off the far-north Queensland coast, home for the past 24 years.

The giant croc, measuring just under 5.5 metres and weighing close to a tonne, lives at Marineland Melanesia's crocodile park, where he has been in the care of owner George Craig for all his captive life.

The two have developed a working relationship in that time, and perhaps even a sort of mutual respect.

"You've got to have a croc mind for a start," Mr Craig said.

"Cassius is a reptile, he's cold-blooded, and he's pretty cold-blooded if I can put it that way."

He said their relationship was such that he could probably swim in the water in Cassius's enclosure without being attacked, but he is not trying it any time soon.

"I like life too much," he said.

Remarkably, Cassius would have been even longer if the 100-year-old croc had not lost an estimated 20-centimetre piece of his tail in a fight at some point in his pre-captivity life.

Mr Craig has passed much of the day-to-day running of the park to his grandson Toody Scott, and it took both men, and all their wits, to measure the croc when Guinness World Records came calling.

"It took months of planning. We drained the pool, got him out and settled, gave him a great big feed and then over a matter of hours he let us get closer and closer until we could put the tape on him," he said.

Guinness World Records spokesman Chris Sheedy said Cassius was one of a few record holders to be honoured with a double-page spread in the 2012 Book of Records, launched on Thursday.

"We thought this record was so amazing that it deserved its own two-page spread and that's now going out in four million copies worldwide," he said.

However, Mr Sheedy said Cassius's reign as world record holder may be brief.

A larger beast, reputedly measuring between six and seven metres, has been caught in the Philippines.

"But until it's in acceptable captivity which is humane and professional, and until it has been properly measured, we can't accept it," he said.

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AAP