Howard on Thursday refused to rule out further increases to the Australian military presence in Iraq following this week's shock decision to more than double the number of troops on the ground, but said they were unlikely.
"It won't become a bottomless pit," Howard told commercial radio. "I am just exercising proper caution.
"I don't think it is at all likely that we will send any more people, but I am not going to get into this business of giving absolute guarantees and having everything I say on that analysed in the future," he said.
Howard's comments followed his announcement on Tuesday that Australia would send another 450 troops to protect a Japanese humanitarian mission in southern Iraq and to help in the country's transition to democracy.
The taskforce will leave in about 10 weeks to join 950 military personnel in the Gulf, about 400 of whom are stationed in Iraq.
Retired major-general Alan Stretton, the chief of staff of Australia's military forces in Vietnam from 1969 to 70, said the situation in Iraq was becoming increasingly similar to the conflict in the Southeast Asian nation.
"I really believe it will go the same way as Vietnam," Stretton told commercial radio. "It will get no better, only worse.
John Howard: Analogies with
Vietnam are misplaced
"Eventually public opinion in both the US and Australia and elsewhere will demand our troops come back and when they do they will be pretending that the locals can handle it all themselves, and we will just leave a bloody mess."
Stretton said internal ethnic differences meant Iraq would never be democratic in the conventional sense and the political system could not be imposed on the population.
"You have three different people in three virtually different areas. The most you could have would be some sort of loose confederation."
Howard has rejected parallels between Iraq and Vietnam.
"I think these analogies with Vietnam are misplaced and many other people think they are too," he told ABC television late on Wednesday.
"I accept the historical facts about Vietnam. I also know the historical facts about Iraq, and they are totally different situations."