Former foreign affairs minister Alexander Downer is standing by Australia's decision to join the invasion of Iraq five years ago.

Mr Downer said the then Iraqi leader, Saddam Hussein, was a cruel dictator with a program of re-establishing weapons of mass destruction, and deposing him was "the right thing to do".

But he admitted the US-led "coalition of the willing" was underprepared and had made mistakes after the invasion.

"It was the right thing to do, to get rid of a brutal dictator," he said on ABC Radio today.

"He (Saddam Hussein) was the cruellest of all dictators on earth at the time.

"Although weapons of mass destruction weren't found subsequently, we didn't obviously know that at the time five years ago.

"What we did discover was that Saddam Hussein did have a program to re-establish weapons of mass destruction."

Mr Downer revealed there had been a heated debate in Washington at the time of the invasion about the number of troops needed to stabilise Iraq post-invasion.

"With the benefit of hindsight, those who thought there should be more troops were the people who were right," he said.

Peace ‘unlikely soon’

Australian Democrats Leader Lyn Allison says Iraq is in a "shocking" state five years after a US-led coalition invaded the country.

But Australia, which took part in the invasion on March 19, 2003, still had a responsibility to help bring peace to the stricken nation, she said.

"I'm not sure that I could've predicted that it would still be going five years on," Senator Allison told reporters in Canberra today.

"But we all knew that this wasn't going to be a war that you could move in with the shock and awe and bomb the country, destroy its infrastructure, walk away and expect that democracy would make it a safe place to be.

"We all owe a great deal to Iraq to help restore their country, but the signs are not looking good for that being possible any time soon."

Greens 'have always opposed the war' 

Australian Greens Leader Bob Brown said his party had always been against the Iraq War.

"This has been a monstrous mistake by George Bush, John Howard, Tony Blair and the others who were involved in it," he told reporters in Canberra.

"The appalling death toll including tens of thousands, if not hundreds of thousands, of innocent Iraqi civilians is going to keep going.

"It's time for an end to the occupation of Iraq and for the Iraqis to sort out their own future."

Nationals: we can't withdraw now

Nationals senator Barnaby Joyce said it would not be wise to completely withdraw Australian troops from Iraq now.

"We are getting results, we are winning the battle there and we should deliver to the Iraqi people, irregardless of the motivations of why we went in there, ... a sense of balance and democracy."

Family First senator Steve Fielding said the federal government must help the United Nations gain more power in Iraq.

"The UN can play a very big part in making sure that Iraq, long-term, can be made a better place."

During the invasion, Australian special forces operated in Iraq's western deserts while Australian warships assisted in securing Persian Gulf oil facilities and the main port.

RAAF F/A-18 Hornets flew strike missions in support of the invasion force.