A humiliated Victoria Police will re-examine every crime solved using DNA after having to drop murder charges which were based on contaminated evidence.
The charges were on Wednesday withdrawn against Russell John Gesah who was last month charged with the 1984 murders of Melbourne woman Margaret Tapp and her nine-year-old daughter Seana.
The evidence used to solve 7,000 crimes in Victoria in the 20 years since the technology was introduced would be reviewed, Deputy Commissioner Simon Overland told reporters on Wednesday.
"We have reviewed 500 of those to date, we have not found a similar problem in those cases," he said.
"It's obviously an embarrassment and we would rather not be in this position."
Gesah, 43, a prisoner at Fulham Correctional Centre in Victoria's south-east, was paraded before media cameras two weeks ago when he was arrested by homicide squad detectives and charged with the double-murder.
Forensic scientists had found Gesah's DNA on clothing belonging to Seana, who had been raped.
Six days after he was charged police Forensic Services Centre scientists realised on July 28 there was a strong chance the evidence had been contaminated.
On Wednesday, an embarrassed Mr Overland publicly conceded the police had got it wrong.
"Only this morning we concluded testing and have got to the stage where we are satisfied we can't rely on DNA evidence in this case," he said.
The evidence possibly became contaminated when clothing from an unrelated offence involving Gesah was examined on the same day Seana Tapp's clothing was in the same area of the laboratory in 1999, Mr Overland said.
Such contamination would not occur today because of more stringent cleaning processes in line with more sensitive DNA testing systems, he said.
The police have also been criticised for charging Gesah and informing the media, only to look foolish today, forcing changes to how quickly information is released to the public.
"We have very sophisticated systems in place to guard against cross-contamination in the laboratory, the evidence is they work 99.99 per cent of the time, here the .01 per cent has slipped through and been picked up," Mr Overland said.
"We have refined the process to ensure the .01 is picked up before the information is released and passed to the public.
"This is human error, we have systems because humans aren't fool-proof and occasionally make mistakes, but the system picked that up and the public can and should have confidence in the integrity of the DNA testing processes in place."
Police have apologised to Gesah, who could potentially pursue a defamation claim, although Mr Overland would not reveal how the apology took place.
They also apologised to the Tapp family, who he said were "disappointed but understanding".
The bodies of Margaret Tapp, 35, and Seana were found in their suburban Ferntree Gully home on August 8, 1984.