With a fist raised in defiance, military whistleblower David McBride addressed a large crowd of supporters on his way into court.
"Today, I serve my country," he said.
McBride faces five charges of stealing and unlawfully disclosing information about alleged misconduct by special forces troops.
All sides acknowledge the former Australian Defence Force lawyer leaked secret allegations of Australian war crimes.
The prosecution and defence have different views on what legal obligations McBride was under and the protections afforded to him.
Supporters gathered outside the ACT Supreme Court before the hearing begun on Monday, urging the federal government to drop the prosecution.
Federal MP and former journalist Zoe Daniel said it was fundamentally wrong that someone who exposed wrongdoing was facing criminal charges.
"If this man goes to jail for telling the truth, it will be a national shame," she said.
At least 100 supporters gathered outside court, blowing whistles and bagpipes as McBride arrived with his support dog Jake.
Former senator and whistleblower advocate Rex Patrick said the prosecution would discourage others from coming forward.
Mr Patrick described the military whistleblower as a hero who called out wrongdoing.
"He told the truth and one of the casualties of this trial may well be the fact that he goes to jail - that's an awful, awful situation."
ACT Greens politician Emma Davidson said the more than $1 million spent pursuing the whistleblower could be better spent elsewhere.
"I firmly believe that this prosecution is not in the public interest," she told the crowd.
"This prosecution needs to be dropped and it needs to be dropped now."
Greens senator David Shoebridge said McBride was brave to reveal the alleged war crimes.
"Now, if that's a crime, then every Australian should be guilty because you'd hope every Australian stepped up in that moment, as a matter of principle, like David McBride," he said.
Monday's preliminary hearing focused on what evidence would be considered during the trial.
The hearing continues and a jury is expected to convene later this week.
