Thomas Chris Lang was convicted twice of murdering Mrs Boyce, 68, whose bloodied body was found in her Kangaroo Point apartment with a kitchen knife protruding from her abdomen in 2015.
Lang's first conviction in 2017 over his lover's death was quashed on appeal, but he was found guilty again in November 2020 and sentenced to life behind bars.
The High Court on Wednesday dismissed his latest appeal on two grounds: that the jury verdict was unreasonable and the evidence of forensic pathologist Beng Beng Ong was outside his expert knowledge.
Mrs Boyce was stabbed four or five times while lying in her bed. Only she and Lang were in the apartment when she died.
Queensland's Supreme Court and Court of Appeal were told she was either murdered by Lang or died by suicide.
The High Court judges said Dr Ong's evidence supported the prosecution's murder case and, taken as a whole, the evidence admitted at trial was sufficient for the jury to exclude, as a reasonable hypothesis, that Mrs Boyce died by suicide.
The judges were unanimous in finding the verdict was not unreasonable.
Three out of the five judges also found Dr Ong's opinion – that the wounds were more likely inflicted by another person than self-inflicted – was substantially founded on his specialised forensic pathology knowledge.
Chief Justice Susan Kiefel and Justice Stephen Gageler said Dr Ong's opinion was based on there being four or five blade tracks through a single incision, with what he considered to be partial withdrawal and rotation of the blade after a slight delay.
"The difficulty, which he acknowledged repeatedly, was that this wound did not fit a standard pattern. In his terminology, it was 'odd'," the two judges said.
"What Dr Ong did not do well was explain why he found those features of the wound more consistent with stabbing by someone else than stabbing by the deceased."
Justices Michelle Gordon and James Edelman, who dissented over the second appeal ground, said there should be another retrial.
They said although evidence showed the injury may have taken up to five seconds to inflict, the significant blood loss occurred as a result of a much slower seepage.
"Mrs Boyce would likely have been conscious for between five and 15 minutes from the time the wound was inflicted, experiencing a physiological fight or flight response if she had been stabbed by the appellant.
"But the scene was not consistent with any movement or a struggle... there was no evidence that Mrs Boyce attempted to make a phone call from the landline phone beside her bed."
The two judges said it was not put to any of the expert witnesses nor suggested in any appeal submissions that this could be explained by Mrs Boyce experiencing a physiological "freeze" response.
While they accepted the jury verdict was not unreasonable, Dr Ong's evidence on the "critical issue that went to the very heart of the matters in dispute" should have been excluded.
Lang and Mrs Boyce had known each other for 30 years and had rekindled an earlier relationship.
The jury in the second murder trial found Lang had stabbed Mrs Boyce in a jealous rage after he read text messages between her and another man.
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