"If America retreats in Iraq, then that has enormous consequences for the stability of the Middle East and it will also be an enormous boost to terrorism in our part of the world," he said in Sydney.
But Howard said he did not "see any need at the present time" to add to his country's 1,300 troops in and around Iraq.
The US president announced on Wednesday the deployment of 21,500 additional US troops to Iraq while making a rare acknowledgment of past failures in his policy.
Japan withdrew its 550 non-combat ground troops from southern Iraq last year, but has continued air support and Abe has made firm support for Bush a cornerstone of his foreign policy since taking office in September.
South Korea has 2,300 troops in the northern Iraqi city of Irbil to support the US-led reconstruction of Iraq.
Seoul's contribution of forces makes it Washington's biggest coalition partner after Britain. Seoul plans to withdraw 1,100 troops by April, and parliament has instructed the administration to devise a full pullout plan in 2007.