CANBERRA - The United Nations is investigating the deportation of an Iranian-born Muslim cleric, who must leave Australia within six weeks without being told the reasons for the order.

Iranian-born Sheikh Mansour Legahei has been told only that he must leave the country on national security grounds after an "adverse assessment" by the domestic spy agency, the Australian Security Intelligence Organisation.

The order has outraged supporters, including a campaign led by an Anglican priest, civil rights groups, and community organisations which have described him as a force for good.

Legahei's wife, Marzeieh Tabatabaei Hosseini and one Iranian-born son, Mohammed, have been granted permanent residency after intervention by Immigration and Citizenship Minister Chris Evans.

Three other children are Australian citizens.

Leghaei's deportation has been confirmed by Evans following a series of hearings and appeals before the Immigration Review Tribunal, the Federal and High Courts.

His appeals failed after a judicial ruling that national security considerations overruled natural justice or procedural fairness.

Last month the UN wrote to Evans seeking a one-year delay in the deportation to allow it to complete an investigation of a case it believes may involve a breach of human rights.

"If Australia proceeds to deport the Sheikh it will be in direct contravention of its obligations under its international human rights treaties and in direct contravention of a specific order issued by a UN committee," Associate Professor Ben Saul, co-director of Sydney University's Centre for International Law, told ABC radio.

"He's been denied an opportunity for a fair hearing.

"International human rights law requires that a person be given sufficient information or access to the evidence against them before they're deported on national security grounds. At no time has the Sheikh been able to see any of the evidence against him."

Leghaei, who arrived in Australia on a work visa in 1994, fell foul of Australia's anti-terror laws that have handed ASIO wide powers.

The agency's powers have been progressively tightened since the September 11, 2001, attacks against the United States and the Bali bombings the following year.

Warnings that the country is at threat from extremists within Australia have been confirmed by the convictions of members of cells planning attacks in Melbourne and Sydney.

But Leghaei is regarded as a moderate Shiite whose sermons at Sydney's Imam Husain Islamic Centre are frequently filled to capacity, and who has been lauded both from within and without the Muslim community for his work with youth and his promotion of harmony between religious and ethnic groups.

He was given character references by Attorney-General Robert McClelland and a predecessor, Phillip Ruddock, has a support group led by Anglican priest David Smith, and has the backing of other Christian and indigenous groups.

Leghaei has been told only that he is suspected of "acts of foreign interference", an allegation he denies.

"This is the mystery in this case, that you are thought to be a risk to national security, despite all the positive work that you have done here and despite all numerous letters of support," he told ABC radio yesterday.

But Evans said he had carefully considered Leghaei's submission against deportation and had determined that it was not appropriate for him to intervene.