Australia's military operations chief says it is now a matter of "when, not if" the Islamic State terror group is defeated in Iraq.

In the rosiest picture yet painted of the often-maligned campaign against the militant organisation, David Johnston, the Chief of Joint Operations, said the group was now on the back foot in key fights across the strife-torn country.

Vice-Admiral Johnston, who has just returned from Iraq, said that "momentum had shifted" in multiple strategic points across the country, notably around Sinjar, Baiji oil refinery and the key city of Ramadi, which Iraqi forces hope to retake within weeks.

"If I gave you a sense of it from the discussions that I had in Iraq, the language that is being used now is it's a matter of when, not if, ISIL will be defeated in Iraq," Vice-Admiral Johnston said.