Yet his professional career – and his personal life – will forever be defined by a drug exchange he wishes he had never, by chance, witnessed.

One of the two people he arrested and charged that night in April 2011, Wayne Edward Jones, was a major crime figure who, already serving parole, was sent straight back to jail – where he should have remained for several years.

Yet for reasons known only to a select few officers within the NSW Police Force, he did not.

Six months later, the charges against Jones were inexplicably withdrawn and he was freed - with deadly consequences.

Jones later booked into a Coffs Harbour motel where, high on ice, he hogtied, tortured and strangled to death a mother-of-four, Michelle Reynolds. He then ordered take-away pizza beside her broken body before dumping her in bushland the following day.

Senior Constable Roberts, meanwhile, found himself charged with having fabricated "false evidence" in the drug case against Jones.

A Fairfax Media investigation has now found that the force appeared so determined to discredit the officer over what he saw that night, it broke the law by withholding two crucial pieces of evidence from the Department of Public Prosecutions (DPP) and Senior Constable Roberts' defence lawyers which proved his innocence.

As a magistrate was still getting his head around the prosecution's case against Senior Constable Roberts, which he later remarked "should never have started", the worst possible news surfaced in court.

The same violent offender whose drug supply charges had strangely evaporated 14 months earlier had since become the subject of another serious criminal case at Coffs Harbour.

"Sorry your honour … I just have a question," said a court assistant about what first appeared to be a mix up with files. "The case … is for a murder charge."

"We all looked around in disbelief," recalled Senior Constable Roberts.

"The man whom I had charged, who should still have been inside, and for whom I was now in court, had killed someone. I was absolutely devastated."

On April 4, 2011, Senior Constable Roberts and a colleague were patrolling Sydney's Kings Cross where they observed Jones and three young women in a situation that prompted concerns of underage prostitution. Senior Constable Roberts then observed Jones "clearly and without obstruction" place both his hands down the front of his pants and remove "a plastic item" before transferring the object into the co-accused's hands" which she swiftly stuffed down the front of her shorts.

They called for back up and a a female officer searched the girl and located the package inside her pants which contained bags of heroin, ice and marijuana.

However, back at the station, the seemingly straightforward arrest started to unravel when the 21-year-old woman divulged that she had been assisting Newcastle-based detectives with classified intelligence about Jones and his bikie gang associates, describing scenes involving big silver cases and "pounds of drugs" laid across tables. "He is part of the Nomads ... they all are," she said. The woman went on to explain how the previous evening Jones had rounded her and two teenage girls up, conducted an ice deal at a service station and then bashed her and forced her to drive, unlicensed, to Sydney for the purpose of prostitution.

"He had sexual intercourse with me even though I tried to stop him ... and then after that he forced me to do two jobs …otherwise he was going to do it again." She also alleged he had raped one of the other girls.

Throughout the interview, the woman said she was "scared", adding: "Once he overdosed me on heroin and just left me there. Other days he just belts me."

Years earlier, Jones had smashed a woman so hard with a car "club lock", it caused the left side of her face to collapse. He received a seven and a half year sentence with a non-parole period of four and a half years.

He was still on parole for that horrific attack when the drug exchange took place. He was now served with three drug possession charges, one count of dealing with suspected proceeds of crime and an additional charge of supply of an indictable quantity of drugs, which carries a maximum 15 year prison term.

Yet six months on, some shadowy element in the police force set wheels in motion to withdraw all those charges and have Jones freed.

In turn Senior Constable Roberts was suddenly accused of lying about what he'd observed on the night and was charged with "fabricating false evidence with intent to mislead judicial tribunal".

When the case was heard in Sydney's Downing Centre in April 2013, it emerged that the prosecution's case against Senior Constable Roberts hinged on one statement from a senior constable who said Roberts had told her he "hadn't actually seen" the drug transaction that led to Jones being charged.

Yet two pivotal pieces of evidence, which the force had failed to produce for two years, proved otherwise. The first, an official record of interview in which Jones' co-accused acknowledged she personally saw Senior Constable Roberts witness the exchange. "I know you saw me," she said, adding: "I spotted that."

The second testimony came from the female constable called to the sceneto search the three women. In her statement, which police did not disclose, the officer recalled Senior Constable Roberts saying: "I've seen her hug the accused and possibly put something down the front of her pants."

Under the Director of Public Prosecutions Act 1986, police are legally bound to "disclose" to the DPP "all relevant information, documents or other things obtained during the investigation" that might reasonably be expected to assist the case for the prosecution or that of the accused person.

Magistrate Graeme Curran said it was that "critical" evidence that not only favoured the "truthfulness" and "accuracy" of Roberts' observations, but "founded" the supply charges then laid against Jones.

"For reasons which just remain completely inexplicable and quite strange … this document was not provided to the DPP. This is despite a request that it be made available to the DPP."

He added: "It seems quite exceptional, quite unacceptable, and as far as I am concerned, quite inexcusable in relation to the conduct of this matter before the court."

NSW Greens justice spokesman David Shoebridge said on Saturday: "This was either the grossest incompetence or, these actions were conducted with the clear intent of delivering a serious miscarriage of injustice. Either way, the consequences have been deeply tragic."

Senior Constable Roberts has had plenty of time to speculate on why someone in the force freed Jones and then attempted to "throw him under a train". But central to the grief that still consumes him is the question of what might have unfolded, had he never made the arrest that night.

"I'm still plagued by the thought that I may have saved the lives of those three young girls, but I cost another woman hers."

On Saturday, the force released a statement to Fairfax Media acknowledging "the seriousness of this issue." 

How the bizarre sequence of events unfolded

Feb 2003: Wayne Jones bashes a woman so hard with a car "club lock", the left side of her face collapses. He already has convictions for armed robbery, possession of a pistol and numerous drug-related charges. At the end of the year, he receives a 7year sentence with a non-parole period of 4years.

Apr 2011: Kings Cross Senior Constable Glen Roberts witnesses a drug exchange involving Jones and a woman who he allegedly brought to Sydney to prostitute. Jones' parole is revoked and he is returned to jail. It emerges the woman has been forwarding classified intelligence about Jones' involvement with a major drug supply and the Nomads motorcycle gang.

Oct 20: All charges against Jones are withdrawn. He is freed.

Nov: Within weeks of being released, Jones is charged with possessing a knife in public, driving while disqualified, dealing with proceeds of crime and possessing identity information to commit an indictable offence. He again avoids jail and is placed on good behaviour bonds, the last of which expires on November 18, 2014.

October 10, 2012: Senior Constable Glen Roberts is charged with "fabricating false evidence with intent to mislead judicial tribunal".

December  11-17: Jones tortures, bashes and strangles Central Coast mother Michelle Reynolds in a Coffs Harbour motel room, then dumps her battered body in bushland.

June 6, 2013: A judge dismisses the case against Senior Constable Roberts and is scathing of police after they were found to have concealed "critical" evidence from the DPP that verified the detective's "truthfulness" and the case against Jones.

October 2014: Jones is sentenced to minimum 20 years jail for murder.

Do you know more? eduff@fairfaxmedia.com.au