THE man who helped put one of Australia's worst serial killers in jail has handed back his massive reward.

Paul Onions escaped from the clutches of Ivan Milat and later turned star witness in the trial that would lock the killer up for life.

He told the Sunday Herald Sun he can't remember how long he kept the reward cheque of more than $200,000, but he was always certain he would never cash it.

In his eyes it was blood money, or "hollow money" that would never bring him any joy.

The cheque was a constant reminder to Mr Onions, 43, of his involvement in one of the most terrifying chapters in Australia's criminal history.

On the 20th anniversary of his escape from Milat's clutches on a highway, Mr Onions reveals for the first time how he returned his reward for his crucial role in putting the serial killer behind bars.

"At first they said, 'Do you want the money?' and I was like, 'No, no, no.' Then they wouldn't really let it go and gave me a cheque," the Briton told the Sunday Herald Sun.

Attempting to move on with this life, he walked into the Australian High Commission in London and handed over the cheque in an envelope.

"It was about 10 years ago. I said, 'Just give that back to the Government'."

Mr Onions is not wealthy, but believed that if he took the money, "Then everything you ever buy - it's like false money ain't it? Hollow money. Everything you buy, you don't have any joy in it".

The past 20 years have effectively been a bonus for Mr Onions. As a survivor of a serial killer attack he is a member of a very exclusive and fortunate club.

He has made the most of his good fortune, living life to the full through sports such as skiing and yacht racing.

In 2008, he was a member of the team that won the Clipper Round The World yacht race. But it was almost an experience he didn't have.

On January 25, 1990, Mr Onions was hitchhiking on the outskirts of Sydney, weeks after arriving in Australia on an around-the-world holiday.

He was heading south to go fruit picking and was offered a lift by a man who called himself Bill. Soon into their journey, the driver pulled out a gun and told his 24-year-old passenger it was a robbery. The man was roadworker Ivan Milat.

Mr Onions recalls how earlier that day he had been with a group of former British miners, who told him he'd be back from fruit picking in a week because it was such hard work.

"I ended up back the same day (because of the attack). I walked in the pub and they were all laughing. I was like, 'Look, some f---er just tried to kill me'."

Detailing the attack to a Supreme Court jury, Mr Onions told how Milat tried to tie him up: "I saw the rope and that scared me more than the gun. I undid my seatbelt and jumped out of the vehicle."

Mr Onions ran down the Hume Highway, as Milat fired at least one shot. Mr Onions reported the incident to police.

Years later, he was home when he saw news reports of bodies being found in the Belanglo State Forest. Mr Onions called the Australian High Commission in London and became the star witness at Milat's 1994 trial, which took its toll.

"I was knackered mentally for 10 years," he said. "I'm more at peace now."s, you know, it happened and that was it. What can I do about it? You try your best don't you?"

Asked about theories there could be more than one killer, Onions says he can only speak for his case, in which there was only Milat.

This year, finally, he is considering a return to Australia for the first time since Milat's trial.

"Maybe the next Ashes," he says. "I wouldn't mind going back - it's nearly time.