A US peace activist branded a threat to national security by Australia will not fight deportation, his lawyer has said.
Campaigners have protested against Mr Parkin's arrest
Scott Parkin will take legal action against officials to find out why they decided he was a security risk, his lawyer Julian Burnside added.
Mr Parkin was arrested following a Sydney protest against US military contractor Halliburton, a firm with close ties to the Bush administration.
Opposition leaders have accused the government of pandering to the US.
"I think the big question here is whether it's a political arrest and deportation," said Green Party leader Bob Brown. "It seems to have nothing to do with terrorism."
Australian Attorney General Philip Ruddock told national television that the decision "was based upon a security assessment and security assessments are not matters about which I can comment in any detail".
Mr Parkin had been interviewed on Australian television as a leader of a protest against Halliburton's involvement in Iraq.
He was detained on Saturday in Melbourne and is expected to be deported within the next few days.
The US peace activist had been in Australia since June. His lawyer said he wanted to know why it had taken the authorities so long to act if he was a security threat.
"If all Mr Parkin has done to be assessed a security risk is to peacefully protest his opinions, then we are in serious trouble," said Mr Burnside.
Australian Prime Minister John Howard has been a staunch supporter of the US-led war on terror and Australia contributed troops to the invasions of Iraq and Afghanistan.