Milat had been serving a life sentence for killing seven backpackers between 1989 and 1992 and dumping their bodies in a New South Wales forest.
He died of cancer in a Sydney hospital early on Sunday local time.
Police said Milat's lifelong refusal to admit his crimes had hampered further investigations into the killings and other unsolved cases.
His murder victims were three Germans, two Britons and two Australians. All were aged between 19 and 22.
Milat was arrested after targeting another backpacker, British man Paul Onions, who escaped and alerted police.
A subsequent trial heard that Milat had searched for hitchhikers to abduct from a major highway between Sydney and Melbourne.
The bodies of his victims were found buried in the Belanglo State Forest, 120km (75 miles) south-west of Sydney, in 1992 and 1993.
Milat was diagnosed with terminal oesophagus and stomach cancer earlier this year.