Retired Justice Frank Vincent’s report on how DNA contamination led to the r-pe conviction of teenager Farah Jama ought to be compulsory reading for every cop, lawyer and judge in Australia. It’s a shocking account of how easily dozens of justice officials, from jurors to prosecutors, came to believe a totally ridiculous fiction.

The good news is that Vincent’s report is no whitewash. Instead of the usual reassuring words about the justice system, the ex-judge sounds alarms about a great many things, including the sloppiness of r-pe victim examinations, the hopeless directions given to Australian juries and, worst of all, the almost total ignorance about DNA evidence among the very professionals we entrust to use it.

Vincent firmly declares that no Victorian should ever be convicted again solely on the basis of DNA evidence. DPP Jeremy Rapke and Attorney-General Rob Hulls say that they’ll heed his call (which the High Court may well repeat nationwide in a coming appeal.)

If Rapke and Hulls mean what they say, then there’s a hard road ahead of them. That’s because Jama isn’t the first Victorian to be convicted solely because of DNA. There’s another, and he’s one of the most reviled people in the state.

In May 2007, cop killer Bandali Debs was convicted of the murder of Kristy Harty, who was shot in the head after pr-stituting herself on the Princes Highway. Debs’ DNA linked him to s-men on her body and the Crown’s theory was that Debs opportunistically killed her, before or after s-x. But the non-DNA evidence admitted at his trial was scant indeed. A petrol receipt placed Debs on the same highway (10 kilometres away near his Narre Warren home) three or so hours before Harty was last seen alive. And, three years later, a gun of the same very common model used to shoot Harty was found buried at Debs’ mother’s home in Sydney.

The case for Debs’ innocence isn’t another forensic stuff-up, but something much simpler: the possibility that his DNA was on her body because he had s-x with the teen prostitute and someone else later shot her. After all, Harty had told her mother the previous day that someone wanted to shoot her over a drug debt. While an expert told the jury that the arrangement of s-men stains meant that the s-x and shooting were contemporaneous, there were other explanations, such as the mentally ill victim putting her underpants on backwards after s-x.

Just like in Jama’s case, the weakness of the evidence against Debs was recognised at first but soon ignored. After Magistrate Jane Patrick refused to commit Debs for trial, the DPP simply overrode her. Debs was promptly convicted (by a jury who naturally knew of his other crimes) and an appeal court refused leave to appeal. Rapke was Debs’ prosecutor and Vincent was one of his appeal judges.

Do the hard lessons of Jama’s case mean that Debs — who Harty’s cousin rightly called an “obviously, cruel, nasty, horrible man with no morals at all” — should be cleared of a murder? Unless new, admissible evidence of his guilt has emerged, the answer must be yes. To see what happens if we ignore that dangers of DNA-based convictions just because we don’t like the defendant and think he’s probably guilty, read Vincent’s report.

Jeremy Gans is an Associate Professor at Melbourne Law School.