David McBride was sentenced to at least two years and three months behind bars after pleading guilty to obtaining and disseminating classified information.
Leave to appeal his sentence was granted by the ACT Supreme Court on Wednesday.
Whistleblower and lawyer Bernard Collaery, who had his own prosecution dropped by the attorney-general after revealing an alleged Australian spying operation in East Timor, told AAP McBride's mental health should have been assessed earlier.
McBride took 235 documents, with 207 of them classified as secret, by printing them at different defence facilities and taking them out in his backpack.
The fact McBride held classified documents in plastic tubs in his house and the haphazard way documents were taken showed there was no pattern to his offending and his mental health was an issue, Mr Collaery said.
McBride's mental health came under scrutiny in his trial, with his lawyers arguing he had been impacted by depression and PTSD, for which he was self-medicated.
The leaked documents led to reports about Australian special forces soldiers committing alleged war crimes.
An inquiry later found credible information about 23 incidents of potential war crimes, which involved the killing of 39 Afghans between 2005 and 2016.
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