David McBride has been working on his mental fitness in the 10 months he has spent in jail, since pleading guilty to obtaining and disseminating classified information.
He was sentenced to five years and eight months in prison with a minimum term of two years and three months in May last year.
This means the 61-year-old will remain behind bars until at least August 2026.
His lawyer Eddie Lloyd, principal lawyer at Lloyd Law, says Monday's appeal in the ACT Supreme Court in Canberra, will be against both the severity of the sentence and his conviction.
There's a reasonable prospect the defence could succeed with its argument that the prison sentence was too severe, she told AAP.
McBride received some leniency for his eventual guilty plea but this would have been entered much earlier had he known he couldn't use a public interest defence, she said.
"We feel the court didn't give much credit to McBride for his plea of guilty," she said.
McBride's mental health was also considered by the judge but the court didn't give enough consideration to an intensive corrections order, which would have allowed him to serve his sentence in the community, his lawyer said.
"They are appropriate in circumstances where there is little risk of reoffending," Ms Lloyd said, adding McBride hadn't offended before and was of good character.
McBride's post-traumatic stress disorder also puts him in a vulnerable position in prison, she said, adding her client will seek leave to take his case to the High Court if the appeal is rejected.
McBride took more than 200 secret documents, which led to reports about Australian special forces soldiers committing alleged war crimes in Afghanistan.
Later, an official inquiry found there was credible evidence that 23 incidents involved potential war crimes relating to the deaths of 39 Afghans between 2005 and 2016.
The court rejected an argument that McBride didn't think what he was doing was a criminal offence, as he believed he was acting in the public interest after taking a defence force oath to serve the nation, which included exposing wrongdoing.
ACT Justice David Mossop said there was a significant need to deter the leaking classified information.
McBride believed soldiers were being "vexatiously" investigated for the war crime of murder to allay political concerns about the death of civilians.
There needed to be reasonable suspicion for an investigation into an unlawful killing to be launched and all deaths - even lawful ones - were being investigated, which McBride argued breached the rules, Ms Lloyd said.
Human rights advocates argue it's an injustice that the first, and so far only person, to be imprisoned over war crimes was the man who helped expose the allegations.
Speaking through his lawyer, McBride told AAP he doesn't believe it's lawful to imprison whistleblowers who attempt to report government wrongdoing.
"If it is, the country is in danger - a country without law has no national security as it's already run by criminals," he said.
Ms Lloyd said her client had his ups and downs in prison. He rows 20km each morning and exercises to keep his mental health in check without the support of his support dog, Jakey.
But fights had broken out and cells were regularly raided, and everything was turned upside down and left to the inmates to clean up.
"It's a very destabilising place to be - you've got to be looking over your shoulder at all times," she said.
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