Howard, one of US President George Bush's closest allies, said Iraq was at a turning point after recent elections and the coalition had to support the fledgling democracy.
"The government believes that Iraq is very much at a tilting point and it's very important that the opportunity of democracy, not only in Iraq, but also in other parts of the Middle East, be seized and consolidated," he said on Tuesday.
Howard said it would be devastating if the Iraqi democracy failed.
Japan welcomed Australia's decision to provide security for the historical mission of Japanese troops, who are legally barred from taking part in combat.
"The decision is encouraging for the international community which is working to rebuild Iraq. The government of Japan welcomes and highly appreciates the decision," Japanese Foreign Minister Nobutaka Machimura said.
Australia already has about 950 military personnel in and around Iraq, although only a few hundred are believed to be deployed on the ground.
"The Japanese presence in Iraq ... is a very important one and if it were to disappear then I think that would, both in substance and in symbolism, be a very bad thing"
Australian prime minister
The prime minister said the initial commitment was for a year, with two troop deployments, each spending six months in Iraq. He refused to say whether it could be expanded after that.
Australia had about 2000 military personnel involved in the 2003 US-led invasion of Iraq, but has scaled back its numbers since then.
Howard did not discuss the possibility of expanding Australia's military presence in Iraq when successfully campaigning for re-election last October, and said changing circumstances on the ground since the poll warranted the additional deployment.
He said it was a difficult decision to send extra troops and pointed out any military deployment involved the risk of casualties.
"I know it will be unpopular with many people," he said. "I ask those people to take into account the reasons that I have given."
The Australian leader said the taskforce would consist of a cavalry squadron, an infantry company and a training team.
"The first [task] will be to provide a secure environment for the Japanese engineering and support forces which are making a valuable humanitarian contribution to the rebuilding process," he said.
"The task group will also be involved importantly in the further training of Iraqi security forces."
Japan has about 600 troops in the relatively peaceful southern Iraqi town of Samawa, the first time since the second world war that Japanese troops have been in a country at war.
Howard said more troops were
being sent at Britain's request
Tokyo renounced the right to use force in its US-imposed 1947 constitution and its troops in Iraq have not fired their weapons since the start of the deployment in December 2003.
Howard said Australia was sending the troops at the request of the British government after the Netherlands, which had previously been providing security for the Japanese, decided not to renew its commitment.
The troops would be deployed in about 10 weeks.
The prime minister said Australia's relationship with Japan was crucial to the decision. "The Japanese element of this is quite crucial because Japan is a major regional partner of Australia," he said.
"The Japanese presence in Iraq as part of the coalition operation, albeit of the humanitarian kind, is a very important one and if it were to disappear then I think that would, both in substance and in symbolism, be a very bad thing."
US marine killed
Meanwhile, a US marine was killed in action on Monday in the western Iraqi province of al-Anbar, the US military said on Tuesday.
"A marine assigned to 1st Marine Expeditionary Force was killed in action yesterday, while conducting security and stability operations in the al-Anbar province," a statement said.
The death brings to 1473 the number of US military personnel killed in Iraq since the US-led invasion of March 2003.