The government won’t discuss Australian airstrikes in Iraq before they happen for fear of telegraphing plans to the terrorist group Islamic State.

But Defence Minister David Johnston is quietly optimistic about how quickly the international coalition can degrade and disrupt the spread of IS in Iraq and Syria.

Federal cabinet has approved Super Hornets to start bombing raids against IS extremists in coming days, supported by 400 RAAF personnel.

Senator Johnston wouldn’t give any details about the timing of the Australian airstrikes.

“We would not want to send any forewarning or advice as to not just our operations but coalition operations generally,” he said.

Australia’s engagement also involves about 200 special forces members training and advising Iraqi forces, but they are awaiting final legal approval before deploying.

Senator Johnston said that approval was delayed because of an Islamic religious holiday over the weekend but that didn’t affect the legal capacity to conduct bombing raids.

He wouldn’t put a timeline of how long Australia would be involved in this new engagement in the Middle East.

However, after talking to military leaders, he says they are optimistic about how quickly it can be done.

“We must say months and more because we want to under-promise and over-deliver here,” Senator Johnston said.

Defence will give regular public updates on how operations are going.

Australian forces are part of a US-led coalition that involves Middle Eastern countries as well as western allies.

But Australia can unilaterally refuse to hit targets assigned to it if the risk to civilians is too high, the minister said.

He said Australia’s risk assessments and operations were focused almost exclusively on not killing civilians, a lesson learnt from the long war in Afghanistan.

“All of us who have been there know the secret to a counter-insurgency is to get the bad guys,” he said.