Australian troops have begun heading to the Middle East as Australia embarks on a military mission against Islamic State terrorists that is likely to cost about $500 million per year.

Australian fighter jets and about 600 troops and support staff, including special forces soldiers, will be based in the United Arab Emirates on standby for action in Iraq.

Prime Minister Tony Abbott said some of the contingent were already on their way, and more would follow in the next four to five days.

The cost is likely to be in the hundreds of millions but, according to Mr Abbott, it "shouldn't" jeopardise the budget surplus.

"It's envisaged the commitment in Iraq will cost about the same as our commitment in Afghanistan cost," he said.

"That was in the order a quarter of a billion dollars every six months, but the final costings are obviously yet to be worked out."

The Government has said it would be "absurd" to put an end date on the operation, with Mr Abbott predicting that it could take "many, many months".

The Prime Minister said the special forces soldiers would act as military advisers within the battalion headquarters of Iraqi or Peshmerga forces.

They will be armed but he said they would not go into battle.

"We're not conducting combat operations independently," he told Fairfax Radio.

"They'll be there to try to ensure that those forces are acting effectively against their opponents."

He said the operations would be "highly hazardous".

"There is no risk-free way to engage in combat operations," he said.

However, defence experts have cast doubt on Mr Abbott's assertion that military advisors would not be involved in battle.

Peter Jennings from the Australian Strategic Policy Institute said that Australian military advisers and mentors worked down to the platoon and squad level in Afghanistan, often near or at the front line.

"Staying in battalion headquarters can provide some level of mentoring, some level of advising but really what's critical here is to steel the resolve of Iraqi troops on the ground as they make contact with the enemy," he told the ABC's Capital Hill program.

And given the support of the Iraqi government for the international military action, Mr Abbott said he preferred to use the word "mission" rather than "war".

"Obviously we've sent this force to the Middle East with an eye to engaging in combat operations inside Iraq, but we're not fighting another country, we're fighting the terrorists of the ISIL death cult," he said, using an alternative name for Islamic State.

"I don't want to minimise the difficulties and the dangers here ... because there are difficulties and there are dangers. But I think strictly speaking it's best described as a mission, rather than a war."

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Mr Abbott said he was "not aware of reports" about a US strike on an IS target near Baghdad - a strike that marks an escalation in the scope of the mission.

Troops could be sent in within the next week

A further decision to send forces to Iraq is expected to be taken by the national security committee of Cabinet within the next week.

The Prime Minister said the decision would be based on an "overall national interest assessment".

"These things are constantly subject to review and by the time any final decision to engage in combat operations is taken, there will be very strict rules of engagement, there'll be an absolute plethora of legal documents in place regarding our operations inside Iraq," he said.

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Mr Abbott has been governing the nation from remote north-east Arnhem Land, where he and government officials have set up camp.

On Tuesday morning he had a security briefing via a secure line with advisers in Canberra.

"It is very easy given the wonders of modern technology to do all these things even from a relatively remote location," he said.

His visit was due to end on Friday but the Prime Minister said he may have to cut that short.

"It's possible that I may have to leave a little earlier but, certainly, I intend to have a very deep and full engagement up here."

Shorten insists Labor united in support for action

Labor leader Bill Shorten has backed the Government's decision to prepare for combat, but West Australian Labor backbencher Melissa Parke has broken Opposition ranks, telling the ABC she has concerns.

"I am very concerned that the Government has so quickly escalated Australia's involvement in Iraq from strictly humanitarian protection to now committing Australia to what appears to be an open-ended war that will only increase the security threat to Australians," she said in a statement.

Mr Shorten insisted his party was united and defended Ms Parke's statements as "expressing anxiety".

"I understand the anxiety of Australians about the commitment of Australian Defence Forces into harmful and dangerous situations," he said.

"So I think expressing anxiety is entirely legitimate."

Mr Abbott has thanked the Opposition for its "constructive" approach.

"I can fully understand why Australians, including some members of Parliament, are anxious about anything that looks like Australia reaching out to this conflict but the point I keep making is that this conflict is reaching out to us," he said.

"We might not want to get involved but, like it or not, they want to involve us."