Mr Assange is facing espionage charges in the United States and remains in London's Belmarsh prison, held there since 2019 while fighting extradition proceedings.
At a Hobart rally in support of the detained Australian's release, independent MP Andrew Wilkie said it was "unconscionable" for the US to be pursuing Mr Assange.
The Tasmanian MP said through WikiLeaks, Mr Assange had revealed war crimes committed by the US in Iraq.
"Twenty years has not diminished the horror of the Iraq War," he said.
"It's only because of Mr Assange and WikiLeaks that the world knows of some of the shocking war crimes committed by the United States in Iraq, and for the US to be pursuing him the way they are is simply unconscionable."
Before entering parliament, Mr Wilkie was a senior intelligence analyst within the Office of National Assessments.
He resigned a week before the war because he believed the Howard government was moving with the US based on false allegations that Iraq had weapons of mass destruction.
"Twenty years has not diminished the horror of the Iraq War ... nor has 20 years righted the wrong of the staggering dishonesty behind the war," he said.
"Regrettably no one in Australia has ever been held to account for this egregious misconduct.
"Moreover, the opportunity to learn from it and to reform war powers - in other words to give the parliament responsibility for deciding to go to war - has been ignored."
Mr Assange has spent more than a decade facing extradition and sought asylum for seven years inside London's Ecuadorian embassy before his arrest.
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