New laws aimed at closing a legal loophole that may have prevented retrials in cases impacted by flawed forensic testing have come into force in Queensland.

The amendments to the state's criminal code, which passed through parliament on December 11, make changes to Queensland's double jeopardy exceptions. 

Double jeopardy laws prevent a person from being retried for a crime for which they have already been acquitted.

However, under Queensland's double jeopardy exception framework, a retrial can be granted if there is "fresh and compelling evidence".

Queensland's forensic lab has been the subject of two inquiries in recent years. (ABC News: Stephen Cavenagh)

Evidence is considered "fresh" if it was not presented during the previous trial, as long as it would not have been presented to a court by a police officer or prosecutor exercising reasonable diligence.

The new amendments, introduced by Attorney-General Deb Frecklington, make it clear that the "reasonable diligence" test is not failed if a police officer or prosecutor relied on results from the state's trouble-plagued DNA lab.

"The amendments ensure evidence is not excluded from being fresh in the context of past deficient DNA-testing practices if the police officer or prosecutor did not interrogate the results or request further testing," the attorney-general told state parliament when introducing the changes.

"This amendment puts beyond doubt and ensures that offenders are not prevented from being retried as a consequence of past deficiencies in testing practices."

More than 100,000 DNA samples had to be retested after two inquiries found "deficiencies". (ABC News)

The "deficiencies" at Queensland's forensic science were uncovered following two commissions of inquiry in 2022 and 2023, which found the lab had been deeming evidence samples as "insufficient" despite the fact that further testing may have produced a DNA profile.

More than 100,000 samples needed to be retested, relating to thousands of cases — including rapes and murders.

"Police officers and prosecutors should have been able to rely on the advice of forensic services provided by Forensic and Scientific Services or Forensic Science Queensland," Ms Frecklington told parliament earlier this month.

"The fact is that the amendment is necessary because of the DNA debacle."

'Last chance to get it right'

The 2013 murder of Shandee Blackburn is perhaps the best-known case affected by problems at the Queensland lab, and it was a re-examination of the files in Shandee's case that revealed them.

Shandee's mother, Vicki, and sister, Shannah, said they were concerned the loophole could have been used to prevent victims' families from getting the justice denied to them by testing errors.

Shannah Blackburn said it would allow the justice system to focus on new evidence and prevent cases from being tied up in court, arguing about whether a police officer should have questioned lab results.

Vicki Blackburn is still fighting to see her daughter's killer brought to justice. (ABC Tropical North: Hannah Walsh)

"This is about Queensland victims and any future victims," Ms Blackburn said.

"If these issues weren't fixed and changed, then there was no potential for justice for victims in the future either."

However, for a retrial to be considered, new evidence needs to be uncovered during the retesting of samples.

Vicki Blackburn said she was troubled by a report from earlier this year that found there was "systemic contamination" at the forensics lab, and that questionable testing methods were still being used.

The report also noted there were 13,000 samples that still needed to be retested, a process that could take years.

"Unfortunately, I think we're further behind than where we were at the time of the first inquiry (in 2022)," Vicki Blackburn said.

"We can only hope that it will get there because this is our last chance to get it right."

"We know that there are amazing scientists at that lab who want to do the right thing, who want to see these results as well," Shannah Blackburn said.

"There's still a long way to go before we can have confidence in the results."