Cassius, the largest saltwater crocodile in captivity, has died.

The 5.48-metre reptile, who lived on Green Island in the Great Barrier reef, was believed to between 30 and 80 years old when he was captured in the 1980s.

He was named the world record holder for largest crocodile in captivity in 2011.

Cassius, weighing in at more than one ton, had been in declining health since Oct 15, Marineland Melanesia Crocodile Habitat said on Facebook.

"He was very old and believed to be living beyond the years of a wild Croc," according to a post by the organisation, based on Green Island near the Queensland tourist town of Cairns.

"Cassius will be deeply missed, but our love and memories of him will remain in our hearts forever."

The group's website said he had lived at the sanctuary since 1987 after being transported from the neighbouring Northern Territory, where crocodiles are a key part of the region's tourist industry.