Not entirely true. People didn't believe Iraq had nuclear weapons, or were even approaching any kind of capability. Even the US didn't believe it, which is why they couldn't convince anyone of it. Everyone believed Hans Blix when he said there was nothing.
However, everyone did believe Iraq had chemical weapons, they had used them in the past. The problem there was everyone had "known" this since the '80s, so it was hardly a reason to suddenly go to war with them.
Whether Australia was involved or not carried little weight. Just a little disappointing. However, without the backing of the UK, would the US have gone ahead? The UK have a large responsibility in this. Despite repeated requests from the UK, the US would not get involved in WW2 until Pearl Harbour was bombed, and they were left with little choice. Why were the UK so quick to support the US? Hard to say, except that England has been the target of many a terrorist attack, and could empathise.
If the reasons are financial or some vague political agenda, then involvement would seem almost criminal. There were always very strong indicators that the US were/are deciding what to do, then trying to prove a case. This of course is backwards, but I sometimes think that some of these guys believe their own hype.