Labor had demanded a new United Nations resolution calling for the invasion, but accepted the assertions Iraq had stockpiles of WMDs, used as the legal justification for joining the war.
Howard's statements that intelligence clearly stated the threat posed by Iraqi WMDs have been challenged by people involved in framing assessments at the time, or in later inquiries into the intelligence provided to the Government.
In his speech to the Lowy Institute Howard said claims that Australia had gone to war based on a lie were "the most notorious one of all about the conduct of my Government, and of others, and merit the most emphatic rejection".
"Not only does it impugn the integrity of the decision-making process at the highest level, but also the professionalism and integrity of intelligence agencies here and elsewhere.
"Some of their key assessments proved to be wrong, but that is a world away from those assessments being the product of deceit and/or political manipulation."
Howard said Australia's belief at the time that Iraq had WMDs was "near universal" and that even critics of the decision to join the invasion, ranging from former French President Jacques Chirac to former Labor leader Kevin Rudd, shared that view.
He cited the inquiry by former diplomat Philip Flood into the intelligence provided to the Government, which found there was a "wealth of intelligence" on Iraqi WMDs, albeit only circumstantial evidence.
The Defence Intelligence Organisation told the inquiry Iraq probably retained a WMD capability - even if degraded over time - and kept both an intent and capability to relaunch a wider programme when possible.
Howard said the Flood inquiry found "no evidence of politicisation of the assessments on Iraq either overt or perceived" and that neither it nor a parliamentary inquiry backed claims the Government had lied.
"Neither gave a skerrick of support to the proposition that members of my Government had manufactured convenient intelligence or strong-armed the agencies into saying things they did not believe," he said.
But Howard's claims were disputed in an article in the Sydney Morning Herald yesterday by Margaret Swieringa, a retired public servant who was secretary of the federal parliamentary intelligence committee when it drafted its report on intelligence on Iraq's WMDs.
In the run-up to the war Howard had said former dictator Saddam Hussein had an arsenal of stockpiled WMDs and a huge programme for developing offensive biological weapons that was larger and more advanced than before the 1991 Gulf War.
"None of the Government's arguments were supported by the intelligence presented to it by its own agencies," Swieringa wrote. "None of these arguments were true."
She said Howard had been "selective to the point of being misleading" in his quotations from the findings of the parliamentary inquiry.
It had found instead that intelligence assessments reported that the threat from Iraqi WMDs was lower than a decade earlier, its military capability was limited, its nuclear programme was unlikely to be far advanced, it had no nuclear weapons and there were no ballistic missiles capable of hitting the US.
Assessments further said there was no known chemical weapons production or specific evidence of a resumed programme, no known biological weapons testing or evaluation since 1991, and no evidence that chemical weapon warheads had been developed for ballistic missiles.
The committee concluded that the Government's case for war based on WMDs "is not the picture that emerges from an examination of all the assessments provided to the committee by Australia's two analytical agencies".
Earlier, Howard's justification for the war had also been challenged by others involved in assessing Iraq's WMD capabilities.
Rod Barton, a former United Nations weapons inspector and former director at the Defence Intelligence Organisation, told ABC radio he did not understand Howard's contention that he had been given "a very strong intelligence assessment" that Iraq had a WMD stockpile.
"I'm not going to call him a liar but I don't know why he said that."
Independent MP Andrew Wilkie - at the time of the invasion working with the Office of National Assessments, the key intelligence adviser for Australian Prime Ministers - said the war had been "unethical, unnecessary and illegal".
