Shaikh Taj al-Din al-Hilali will not arrive in Baghdad before the fighters' deadline on Tuesday for Australia to withdraw its troops from Iraq before they kill captive Douglas Wood.
"He's (al-Hilali) hoping [the fighters] have heard his call and the deadline will be extended," Kayser Trad, a spokesman for Australia's Islamic community, said.
Al-Hilali made a televised appeal in Arabic to the fighters holding Wood, a 63-year-old Australian engineer who lives in California and is married to an American. The appeal was broadcast on Aljazeera over the weekend.
"We are going to Iraq to help our Australian brother. We feel for Australians and we will do our best to bring our brother, Douglas Wood, home," al-Hilali said at Sydney airport before leaving.
Al-Hilali is the 64-year-old Mufti of Australia's 300,000 Muslims, who make up about 1.5% of the nation's 20 million population.
He made headlines in 2003 when he was charged with assaulting and hindering police and resisting arrest after his car was pulled over. The charges were later dropped.
More Australian troops are soon
to be deployed in Iraq
Egyptian-born al-Hilali was under the spotlight again in 2004 when he described the 11 September 2001 attacks on the United States as "God's work against the oppressors". He later said his statement had been misinterpreted.
Australian Foreign Minister Alexander Downer has said Wood may have been seized from his Baghdad apartment up to two days before a two-minute video was delivered to news agencies in Baghdad eight days ago.
That video showed Wood pleading at gunpoint for Australia, Britain and the US to withdraw troops from Iraq.
The fighters released a second video on Friday demanding Australia start withdrawing its troops within 72 hours.
Al-Hilali told the fighters in his televised appeal that he valued their jihad, but called upon them to release Wood for the sake of the Australian society, "which does not support [Prime Minister John] Howard's pro-American policies".
Australia's conservative government, a staunch US ally that was among the first to join the war on Iraq two years ago, has stood firm on its refusal to give in to the fighters.
A new batch of 450 Australian troops is due to arrive in southern Iraq in the coming weeks to provide security and train the Iraqi army, taking the total number of Australian troops in and around Iraq to about 1400.
Opinion polls taken in May last year indicated that nearly two-thirds of Australians believed the war on Iraq was unjustified. Half of Australians believed it was not worth sending troops to Iraq, while 40% backed the decision.
Howard won a fourth straight term last October by crushing centre-left opposition Labour, whose leader had vowed to bring Australian troops home by Christmas.