For the man behind the high-profile investigation of Australia's notorious backpacker killer, this was his "gotcha moment".

More than a decade after former NSW Police Assistant Commissioner Clive Small and his task force put Ivan Milat behind bars, Mr Small faces the violent criminal during a visit to a maximum security prison in 2005.

Milat is aggravated at accusations his sister, Shirley, was involved in the infamous backpacker murders in the Belanglo State Forest.

When a grey-haired Milat confronts Mr Small during their chance encounter, Mr Small assures him he knows Milat killed the travellers himself.

The convicted killer replies: "Yes, so why are you telling them she was involved?"

Mr Small has revealed the moment he says is as "close to an admission of guilt" as he ever got from the Southern Highlands murderer in his latest book, Milat.

"As soon as he finished speaking his whole demeanour changed," he writes.

"As if he suddenly realised the significance of what he had said."

Ivan Milat has maintained his innocence despite his conviction in 1996.

Mr Small says the admission was enough to reinforce his investigating team's painstaking police work.

"You got the feeling of `ah ha'! It was that gotcha moment," Mr Small told AAP.

This May will mark 20 years since Milat was arrested after seven hitchhikers' bodies were found in the Belanglo State Forest, south of Sydney.

The victims had been shot, stabbed and blindfolded, and some had been sexually assaulted.

Clothing with slash marks, rags and a garrotte found nearby signified the unimaginable horror these innocent backpackers faced and which Milat was found guilty of inflicting.

However, Mr Small's book also reveals how an internal police conflict threatened to bring the backpacker murder case to it's knees.

In 1992, the bodies of missing 22-year-old backpackers Joanne Walters and Caroline Clarke were uncovered in the forest.

Over a year later, the remains of Deborah Everist and James Gibson were found in the area.

The bodies of overseas travellers, Simone Schmidl, Gabor Neugebauer and Anja Habschied, were discovered a month later.

Missing person reports had been made for the seven backpackers, including by their worried parents overseas.

A huge search of the challenging Belanglo terrain ensued after the grisly discoveries and would cover 78 kilometres of fire trails and involved 300 police officers.

Rumours about the Milat family crossed the police radar early in the investigation, Mr Small says.

But when Englishman Paul Onions called police in 1993 and reported he escaped an abduction attempt a few years earlier by a man matching Ivan Milat's description, Milat became the prime focus.

"It was a tremendous step forward," Mr Small said.

"It showed we actually had evidence, to charge him over the Paul Onions matter ... it showed very strong indication that he was a good suspect in the backpacker murders."

Police later raided Milat's home and found property connected to the dead backpackers.

As police built the case against Milat for the murders, an internal distraction in 1994 threatened the prosecution of Milat.

Former detective Paul Gordon was kicked off the task force after leaking information to the press, prematurely linking Milat with the seven backpacker bodies, Mr Small says.

The press could have given Milat reason to argue he would not receive a fair trial, Mr Small feared.

After Milat's conviction for the murders in 1994, Mr Gordon's media profile continued and he went on to claim Milat wasn't acted on early enough and criticised the investigation.

Mr Small does not hide his dislike for Mr Gordon's behaviour, either in his book or in conversation.

He said he considered suing a current affairs show for airing Mr Gordon's claims - confident he would win - but opted to "grin and bear it" as it risked giving Milat grounds to appeal his conviction.

Milat's impact stretched beyond his conviction in 1996.

In 2010, his nephew Matthew Milat killed a 17-year-old with an axe in the Belanglo forest.

According to evidence presented in court, Matthew told a friend: "You know me, you know my family, you know the last name Milat."

While police take comfort knowing Milat will never see freedom, there is one death investigators have been unable to pin on him.

Paul Letcher was last seen alive after hitching a lift from Busby to Liverpool in 1987.

Nine weeks later, his body was found near the Jenolan Caves covered in leaves and with five bullet wounds in his skull.

Mr Small said the bullets found near Mr Letcher's body were certainly fired from the same gun used in the Belanglo Forest murders.

But it was not enough to hold up in court.

"He has gotten away with it, but at the same time he is in jail, he will never be released and you get satisfaction from that," Mr Small says.

Milat is serving a life sentence.


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