Backpacker killer Ivan Milat, one of Australia's worst serial murderers, has died of cancer at 74.

Milat died in Long Bay Prison Hospital overnight just months after it was revealed he had been diagnosed with terminal oesophagus and stomach cancer.

In the video above, 7NEWS crime reporter Robert Ovadia with new details on Ivan Milat's death

"Ivan Milat, 74, died at Long Bay Hospital 1 about 4.07am on Sunday, 27 October 2019," a Corrective Services NSW spokesperson told 7NEWS.com.au

"All deaths in custody are subject to a coronial inquest, including deaths resulting from natural causes".

What did Ivan Milat do?

In one of Australia's most notorious criminal cases, Milat was convicted in 1996 of the murder of seven backpackers whose bodies were found in the Belanglo State Forest in NSW in the years after they had disappeared between December 1989 and April 1992.

His victims had been shot, stabbed, beaten, or in one case, decapitated. Six of the seven were probably sexually assaulted.

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They were Britons Caroline Clarke, 21, and Joanne Walters, 22; Victorian couple Deborah Everist and James Gibson, both 19; and German nationals Simone Schmidl and Gabor Neugebauer, both 21, and Anja Habschied, 20.

The seven known victims of Ivan Milat. Credit: 7NEWS

All had been looking for a ride south from the Hume Highway, around Liverpool in Sydney's west.

Milat was arrested in May 1994 at his home in Eagle Vale on Sydney’s southwest fringe, police finding a wealth of forensic evidence linking him to the murders.

How many people did Ivan Milat kill?

After a four-month trial, he was convicted of seven counts of murder and handed seven life sentences.

There's long been speculation there were further victims.

But Milat maintained to his death his innocence over the seven killings, and also never confessed to any other murders, despite numerous police efforts.

Milat will spend his last days behind bars. Credit: AAP/ 7NEWS

In mid May this year, the aged and frail prisoner was taken to Prince of Wales Hospital for tests after a deterioration in his health.

His transfer under heavy guard, from the maximum-security Supermax prison at Goulburn, to the public hospital in the Sydney suburb of Randwick, prompted an outcry.

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It was the first time the public had seen images of the killer since his jailing 23 years ago.

He was pictured, white haired and in a wheelchair.

Ivan Milat photographed in May 2019 leaving hospital. Credit: 7NEWS

After spending several days at the Prince of Wales, Milat was taken to Long Bay Jail, where he was expected to live out his days.

In the months starting from early August, he then made several trips between Long Bay and the Prince of Wales while he underwent chemotherapy.

Previously in a wheelchair, by late September his condition had improved to the point where he was able to shuffle in and out of the hospital, aided by police.