I don't see how howard's advisors managed to deduce that the war was legal.
The resolution which the government's advisors relied on was SCR (security council resolution) 687 which was implemented in 1991 for a completely different purpose, which was to eliminate or reduce iraq's ability to threaten kuwait.
There is nothing to justify the conclusion that when the Security Council was formulating these resolutions it had in mind authorising military action some 12 years later in respect to an entirely different issue.
Furthermore, both SCR 687 and SCR 678 clearly states that even if Iraq did violate its obligation to destroy its chemical and biological weapons (as stated in SCR 687), the UN was to 'remain seized of the matter'. That should've clearly indicated to the govt's advisors that individual nations such as US and Australia were not permitted to take matters into their own hands.
In my opinion, John Howard decided to send troops because he thought it would be best for Australia if they didn't piss Bush off. I'm not going to argue whether that's valid reason or not, but howard's action was clearly illegal under international law, and to say he was ill advised is no defence. Also, when questioned about why he did it amongst widespread public disapproval, he clearly said 'if the public didn't like what we're doing, they'll throw us out at the next election'.
Thus voters who were clearly against the invasion and voting for howard in the coming election are inadvertantly saying to howard and any future government "as long as it's not near election time, feel free to use our military in any way you see fit, even if it's against international law and public opinion."