The defence barrister of former Jetstar pilot and alleged double murderer Greg Lynn has accused a police witness of lying during a Victorian Supreme Court trial.
Mr Lynn, 57, is standing trial for the murders of Russell Hill, 74, and Carol Clay, 73, who went missing after a camping trip in Victoria's remote High Country in March 2020.
He has pleaded not guilty.
Prosecutors have told the court Mr Lynn murdered the pair of retirees while lawyers for Mr Lynn have said the deaths were accidental and occurred over two separate struggles over a knife and a gun.
Both prosecution and defence agree that the former airline captain set fire to the couple's campsite at Bucks Camp, in the Wonnangatta Valley, before moving and later burning their bodies.
On Thursday, Mr Lynn's lawyer Dermot Dann KC grilled a police expert witness who had provided the court with evidence about a range of forensic tests he had conducted.
Lynn's barrister grills police witness over 'vacuum theory'
In a fiery exchange, Mr Dann accused Victoria Police forensic officer Mark Gellatly of lying and introducing a "half-baked theory" in the third week of a murder trial.
Mr Dann was referring to evidence Mr Gellatly provided the court on Wednesday regarding what was referred to as "vacuum theory."
"We have a theory, well there's a known phenomenon with firearms, that when they fire, there's a vacuum that's created in the barrel and blood can be drawn back in," Mr Gellatly said on Wednesday.
The court heard that DNA testing was conducted on the interior of the barrel of Greg Lynn's gun, which prosecutors and defence agree was the weapon that fatally wounded Carol Clay.
The tests were inconclusive.
The court heard while "a very partial" DNA profile was found inside the barrel of the gun, it was not enough for any comparisons and the staining on the interior of the barrel could also be explained by gun oil.
Mr Dann put to Mr Gellatly that he did not have the correct expertise to bring up this theory and that he had failed to notify defence that it was going to be raised during his evidence in breach of his obligations as an expert witness.
"What I was doing was, in context, was trying to explain why I was aiming to sample that region. I wasn't trying to be a ballistics expert, because I am not. I was trying to explain why I was targeting that particular area of the gun," Mr Gellatly said.
Mr Dann told the jury that Mr Gellatly had told an earlier court sitting the theory had not been discussed with prosecutors, but then told the jury that "he could not recall" if it was mentioned during his meetings ahead of his testimony.
Mr Dann suggested the difference between Mr Gellatly's responses amounted to "false evidence" or a lie.
Mr Gellatly accepted there was a difference in his evidence but told Mr Dann he "hadn't lied".
Dermot Dann KC: Difference to the truth? If it's not a lie what is it?
Mark Gellatly: I don't know.
Possible blood on Mr Hill's LandCruiser linked to Carol Clay
Before his tense cross-examination, Mr Gellatly told the court how he analysed the rear canopy of Mr Hill's LandCruiser for possible blood in February 2022.
A number of tests were conducted on several stains in the rear compartment of the canopy including on possible stains of blood and "fatty deposits", the court heard.
DNA tests on swabs of each returned "extremely strong" support for a DNA match with Ms Clay.
Tests on the stains that looked like blood were less conclusive, the court heard.
Mr Gellatly told the jury that the substance behaved like blood would and provided a DNA profile but failed one test that would normally indicate blood was present.
"Based on the appearance, behaviour and DNA results, and the presumptive test, I believe it most likely still is blood, and I made an assumption that it is blood, but we couldn't confirm it," he said.
He said that sometimes bloodstains impacted by heat, as the LandCruiser was, could return false negative results.
He also told the court that based on the bloodstain pattern, it appeared some "forceful event" occurred to make the blood spatter in the way it did, if it was indeed blood.
Ms Clay's DNA found on metal fragment recovered from campsite
Mr Gellatly was also involved in DNA testing conducted on a piece of metal recovered from Bucks Camp — nearly two years after the alleged murder took place — in March 2022.
Jurors have previously heard from a different police forensic expert, Tanith Condon, who told the court the lead item was highly likely to be a bullet or shot fragment.
Mr Gellatly explained how the metal item was tested for DNA and there was an "extremely strong" indication — 550 million times more likely than not — that it was a match with Carol Clay.
Mr Gellatly told the court how the item was "accidentally" broken into pieces during the testing process.
Justice Michael Croucher asked Mr Gellatly if he "hit the roof" when the breakage occurred.
He told the Supreme Court judge he was there when it happened, but it wasn't him that broke it.
"Sometimes that happens," he said.
The court heard the prosecution only has one witness remaining, whose evidence has been delayed due to illness.
The trial, which has now wrapped up its third week, will continue on Monday.
