Controversial anti-abortion crusader Troy Newman could be deported at any time after losing a last-minute legal bid.
It is understood the Immigration Department sought to put him on a flight as early as 6pm (AEST) on Friday after the High Court dismissed Mr Newman's application to prevent his deportation.
Immigration Minister Peter Dutton revoked the American's visa over concerns his Australian speaking tour could incite community harm.
Mr Newman flew to Australia anyway and was detained at Melbourne airport.
"His detention and removal is entirely related to his decision to openly flout Australian law and travel to Australia without a valid visa," Mr Dutton said on Friday.
His wife Melissa Newman - whose visa was not cancelled - declined to comment outside court.
Mr Newman had planned to hold speaking arrangements at Right to Life events in Melbourne, Sydney, Hobart, Brisbane and Cairns.
Right to Life Australia's Mary Collier told reporters outside court on Friday her organisation was "very disappointed about the decision".
"We were very much looking forward to hear Troy speak in Australia," Ms Collier said.
Mr Dutton said Mr Newman had been treated in the same way as anyone else who arrives illegally.
He has ordered his department to find out how Mr Newman was able to board a flight despite being on an alert list.
"Any airline involved faces a fine for carriage of a person who does not hold a valid visa," Mr Dutton said.
Justice Geoffrey Nettle ruled the US citizen knew he had to obey Australian immigration laws before boarding a United Airlines flight but chose not to.
"He does not come to this court with clean hands," Justice Nettle said on Friday.
He said Mr Newman had to pay the government's legal costs.
Mr Newman can still get into Australia as a hearing into whether Mr Dutton has the power to revoke his visa is due on October 30.
Labor MP Terri Butler, who opposed Mr Newman's visit, said cancelling his visa does not affect freedom of speech.
"There's a difference between freedom of speech and advocating extremism," Ms Butler told reporters.
Mr Newman has suggested doctors who abort babies should be executed and has been arrested for participating in unruly anti-abortion protests in the US, but has never been charged.
