Lynn, 59, was brought to the Court of Appeal in Melbourne inside a prison van on Friday morning for his much anticipated challenge of a murder conviction and 32-year sentence.
He wore glasses, a suit and tie and clung onto his notebook as he was escorted into the dock by custody officers, in front of a packed courtroom of journalists, authors and observers.
In June 2024, a jury delivered a split verdict and convicted Lynn of the murder of Carol Clay, 73, but found him not guilty of the murder of her secret lover Russell Hill, 74.
Mr Hill's wife Robyn and his daughters attended Friday's hearing, sitting at the back of court behind Lynn.
The pair had been camping at the same remote site as Lynn, Buck's Camp in the Wonnangatta Valley, Victoria's high country, when they both went missing in March 2020.
Lynn was charged with two murders and took the case to trial, admitting he burned their bodies but maintaining the deaths were accidental.
He told the Supreme Court jury he struggled over his shotgun with Mr Hill when it accidentally discharged, and shot Mrs Clay in the head, and that Mr Hill died after a struggle with Lynn over a knife.
Lynn said he was trying to defend himself from Mr Hill when the knife went into his chest, and the jury was not shown any evidence about how Mr Hill died.
The former Jetstar pilot admitted he burned the couple's campsite, put their bodies inside a trailer and drove to the Union Spur Track where he covered them with sticks.
He returned twice, including in November 2020 when he burned Mr Hill and Ms Clay's remains into more than 2000 fragments and scattered them under a fallen tree truck.
Lynn was jailed for 32 years, with a non-parole period of 24, in October 2024.
His legal team flagged they would appeal both the jury verdict and sentence, claiming the prosecution had conducted the trial unfairly and there were inconsistencies in the jury's split verdicts.
Victoria's Director of Public Prosecutions, Brendan Kissane KC, is representing the prosecution as they oppose the appeal before Justices Karin Emerton, Phillip Priest and Peter Kidd.
Lynn's barrister Dermot Dann KC opened the appeal on Friday by claiming trial prosecutor Daniel Porceddu "played outside the rules".
"The prosecution broke the rules from start to finish," he told the court.
Mr Dann said Mr Porceddu had "chickened out" when he questioned Lynn as he gave evidence to the jury, as he failed to put all post-offence incriminating conduct to him for a response.
"He didn't want to come off second best … and he deliberately refrained from putting all sorts of matters to Mr Lynn," the barrister said.
"The incriminating conduct issue and his explanations were vital.
"If the jury rejected his explanations it was a direct pathway to guilty, so the failure to put that whole case to him ... was very very significant."
He accused the prosecutor of breaching cross-examination rules six times, in failing to give Lynn the opportunity to respond to evidence in the case.
Mr Dann argued the jury arrived at an improper pathway or engaged in an illegitimate compromise.
"There has been such a departure from proper processes at trial, that there has been a substantial miscarriage of justice," he said.
The appeal hearing continues.
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