Defence Minister David Johnston is optimistic about how quickly the mission against Islamic State can be done but won’t put a timeframe on involvement.

The first Australian airstrikes against Islamic State extremists are imminent and neither side of politics wants to put a deadline on the military involvement.

Nevertheless, after talking to military leaders, Defence Minister David Johnston is quietly optimistic about how quickly the international coalition or forces can degrade and disrupt the spread of IS in Iraq and Syria.

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

“But putting a timeline on it is very much in the realms of looking into the crystal ball at the moment.”

Labor frontbencher Andrew Leigh said clearly Australians would like to see the mission over as quickly as possible.

“But I don’t think it’s particularly helpful to put timeframes on that,” he told reporters.

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 so as not to telegraph the US-led coalition’s plans.

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 this didn’t affect the legal capacity to conduct bombing raids.

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

Although the United States is leading the mission, Australia can unilaterally refuse to hit targets assigned to it if the risk to civilians is considered 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.