A Queensland government-run forensic laboratory refused to test more than 21,000 evidence samples, including almost a third from murder and rape scenes, arguing they did not meet DNA thresholds.
Inspector David Neville, manager of the Queensland Police Service DNA unit, told the Commission of Inquiry into Forensic DNA Testing the samples collected between 2018 and 2021 were ruled "insufficient DNA for testing" and never fully examined.
But the lab did test 1410 of the samples from major crime scenes at the request of police, with 549 samples yielding a DNA profile.
"So, in excess of a 30 per cent success rate (from the 1410 samples). There remain some 7000 samples from major crimes that have not been tested," Insp Neville told the inquiry on Tuesday.
Queensland Police have since formally requested the remaining samples be examined for potential positive results by the Forensic and Scientific Services lab.
Most of the outstanding samples were classified as "volume crime" and primarily collected from less serious or property crime scenes.
Insp Neville said that of the 21,000 untested samples, he estimated about a third - some 7500 - would have been collected from the scenes of murder, violence and sexual assault.
Samples from unsolved sexual assaults that were retested returned a "disturbingly high" 66 per cent success rate, after being previously ruled insufficient for testing.
Insp Neville said once he identified the discrepancy in results as a "real issue", police were instructed to request samples from major crimes be re-tested.
"I told my staff if you get that result (insufficient DNA for testing) for major crimes, just send it back to the lab for testing," he said.
In other evidence, Insp Neville, who holds a master's degree in forensic science, agreed a forensic laboratory paper outlining changes to DNA threshold limits was complex.
"I'll be honest with you, and maybe this is just my intellect, but I didn't completely understand it," he admitted.
Scientists at the lab also suggested the police service didn't fully understand the scientific process being carried out, and the possible implications it would have on the treatment of DNA evidence.
The probe into Queensland's DNA lab comes after the release of an interim report identifying serious shortfalls in testing thresholds.
The inquiry has been told threshold testing changes were first proposed in 2017 as a way to increase testing speed and lower costs.
Managing scientist Cathie Allen and team leader Justin Howes, who pushed for the change, were stood down after interim findings were released.
Insp Neville described Ms Allen as his managing counterpart at the forensic service, and the pair were in regular contact, as he tried to triage the number of samples sent to the laboratory.
The relationship deteriorated over time as he became "uncomfortable" with the tone of communications between the two departments.
"Cathie became quite aggressive or terse and said to me, 'you can either be my friend or my foe, but once you're my foe there is no coming back'," he told the commission.
Insp Neville said he tried to placate Ms Allen, so the departments could work together.
"She said 'no we don't, we're in different government departments' and that other government departments don't get along and still get the work done."
Insp Neville said it was the first time he had experienced that type of confrontation with another government department.
The pair later met to discuss their differences and Ms Allen, he told the hearing, warned the police service not to interfere.
"I have a distinct memory that I was told basically not to interfere or try to influence the operations," he said.
Earlier on Tuesday, laboratory staff members testified to raising serious concerns about changes to the testing regime but their feedback was ignored.
Senior scientist Kylie Rika told the commission the work culture at Forensic and Scientific Services, operating under the Health Department, was "quite toxic".
The hearing continues in Brisbane on Wednesday ahead of the commission's final report due in December.
