Islamic cleric Sheikh Mansour Leghaei, who was ordered to leave Australia after being deemed a security risk, says he has been denied justice.

Sydney's most senior Shia cleric - who has been living in Australia for 16 years - is due to board a flight to Iran tonight after having his request for a permanent visa denied.

He says he has never been given an explanation of the reasons for ASIO's assessment and maintains his deportation is unjustified.

"I'm not the only person that is unfairly treated and deported ... and I won't be the last unless the law is amended and changed," he said.

"Whatever they think and whatever they may have is absolutely wrong.

"There's an error somewhere. We tried our very best to help them and correct that error.

"They didn't let us, simply because the act does not allow us to do so."

Under Australian law, ASIO is under no obligation to tell Dr Leghaei why it suspects him.

Supporters of Dr Leghaei say he is a leader of inter-faith relations between Muslims and Christians.

His lawyers say the United Nations wrote to Immigration Minister Chris Evans to ask that he not be deported, but this request was declined.

Dr Leghaei will fly out with his wife and daughter but leave behind his three sons.