About 330 more Australian troops will begin heading to the Middle East tomorrow as part of a boosted contingent in the fight against the Islamic State (IS) group.

Federal Cabinet has signed off on the deployment, which was first flagged six weeks ago.

Prime Minister Tony Abbott said the troops will be embarking on a two-year mission, with most put to work training Iraqi soldiers.

"The deployment will start tomorrow and we expect that the force will be deployed and operational by the middle of May," Mr Abbott said.

"It is, as I stress, a capacity training mission not a training mission, but Iraq is a dangerous place.

"It is a dangerous place and I can't tell you that this is risk-free."

The Australian troops will be mostly drawn from the army's Brisbane-based 7th Brigade, and will work in tandem with about 100 soldiers from New Zealand.

They will be based at the Taji military complex about 50 kilometres north of Baghdad.

Australia joined the international effort to defeat IS militants in September, with a contribution of six F/A18 fighter jets, a surveillance aircraft, a refueller, 200 special forces soldiers, and 400 military support staff to the US-led mission.

Mr Abbott said while the international military coalition had been effective, "large swathes" of Iraq still remain under IS control.

"Our build partner capacity mission is all about trying to ensure that the legitimate government of Iraq has a trained and disciplined and capable force that understands the rules of armed conflict at its disposal to retake ... the territory which is currently under the control of the death cult," he said.

Mr Abbott said the deployment would be reviewed in 12 months.

When the increased troop deployment was foreshadowed in March, Mr Abbott rejected criticism it represented "mission creep", saying instead it was the "successful execution of the original mission".