"We have over the past few days received some specific information from intelligence and police sources about a possible terrorist attack," Howard said in a televised interview after the lower house of parliament passed an amendment increasing authorities' powers to crack down on suspected terrorists.
Howard declined to give details about the information at an earlier press conference, and said the national terror alert level would not be raised.
Critics quickly suggested the timing of Howard's announcement of an unspecified terror plot was linked to his government's plans to push through a raft of new anti-terror laws that have been labelled a threat to civil liberties.
But Howard said that following the new alert from security agencies, he had obtained the support of political leaders from across the political spectrum to alter existing legislation so that police could act against suspected terrorists even before they had details of a specific planned attack.
"We have decided to alter the existing terrorism legislation to substitute for a current provision which says that in order to prove a charge you have to prove an established preparation for a specific terrorist act with a more general provision providing for a terrorist attack," he said.
After its passage through the lower house, Howard said the new law would go to the government-controlled Senate during a special session on Thursday.
"The government is satisfied on the advice provided to it that the immediate passage of this bill would strengthen the capacity of law enforcement agencies to effectively respond to this threat," he said.
Howard has been a loyal ally in the US-led "war on terror" and contributed troops to the wars in Iraq and Afghanistan. Australian intelligence on Tuesday issued a report raising the spectre of home-grown terrorism.
The new amendment would add further grounds for banning groups and "clarify that, in a prosecution for a terrorism offence, it is not necessary to identify a particular terrorist act," Howard said.
"It will be sufficient for the prosecution to prove that the particular conduct was related to a terrorist act."
The amendment is part of controversial new anti-terror laws that the government drafted after the London transport bombings by British-born Muslims in July that killed more than 50 people.
The full package of new legislation will be introduced in parliament soon and the government hopes to get it passed by Christmas.
The new laws, under which suspects can be detained without charge for up to two weeks, placed under house arrest and fitted with tracking devices, have been widely criticised as infringing on civil liberties.
The Australian spy agency ASIO said in its report to parliament on Tuesday that some Australian Muslims believed there was "a battle between Muslims and infidels".
They felt "a sense of hostility and isolation towards the broader Australian society," with some viewing the US-led invasion of Iraq as an attack on all Muslims, the report said.
The report said that "some of the more extreme individuals ASIO has identified and investigated are Australian-born. Some have participated in terrorist training overseas while others have never travelled abroad."