Australian troops are heading into Iraq to train local soldiers so they can expel Islamic State jihadists - a campaign likely to culminate in a bloody battle for the city of Mosul.

A new training team, comprising 300 Australians and 100 New Zealanders, heads for the sprawling base at Taji, 30km north of Baghdad, in May to begin instructing.

Prime Minister Tony Abbott says the advance of IS forces had been slowed since the beginning of a US-led air strike campaign last year.

"But Iraq's regular forces now require support to build their capacity to reclaim and hold territory," he said.

Although greatly outnumbering IS forces, the Iraqi military performed poorly in their initial encounters last year.

Some units simply evaporated, their officers scattering and leaving the soldiers to be rounded up and massacred.

That allowed IS - which the government refers to by the derogatory term "Daesh" - to seize a vast amount of territory including Mosul, Iraq's second city, and Tikrit.

In many areas, IS wasn't unwelcome. Former Iraqi prime minister Nouri al-Maliki had managed to thoroughly alienate much of Iraq's Sunni population through his relentless favouring of the majority Shia.

The jihadi advance has now been stemmed through the coalition air campaign - including Australia's six Super Hornets - and through the better quality Iraqi units, including the Counter-Terrorism Service, advised since late last year by the 170-member Australian special forces team.

Now the Iraqi army needs to be rebuilt with enough trained soldiers to finish the job. An initial test will be the campaign to oust IS from Tikrit.

"If they can fight and do something positive in Tikrit then I think there's some hope because the big task is Mosul," said former Australian army head Peter Leahy.

Retired Major General Molan, chief of coalition operations in Iraq in 2004-05, said an Iraqi force of up to 10 brigades - a total of 25,000 troops - would be needed and they would have to be trained.

"What Australia can do is conduct this training with the Kiwis in the same location and feed those troops into the brigades," he said.

Mosul is a city of about two million and IS is expected to bitterly resist the upcoming Iraqi government offensive. No timetable for the attack has been set and it may even be pushed out to next year.

Australian troops won't be involved; the government has made clear this is solely a training mission. Many others will be assisting, with a number of other countries including the US and Spain contributing training teams.

Australian troops are good at training foreign soldiers and have done a lot of it.

Over more than half a century, Australian troops have trained Vietnamese, Cambodians, Iraqis, Afghans, East Timorese and even Ugandans.

By far the biggest and longest training mission was in Vietnam, with the Australian Army Training Team-Vietnam first deployed in 1962.

This was to become Australia's most decorated unit of the Vietnam war. Four members won the Victoria Cross - the only VCs awarded in Vietnam.

The unit - which started at 30, peaked at 200 but averaged about 100 - was in Vietnam for a decade. As well as instructing, team members went out on operations and routinely fought and sometimes died alongside their South Vietnamese charges.

Since Vietnam, Australia has mounted a variety of missions. In Uganda in the early 1980s, small teams instructed soldiers in basic infantry skills.

From 1989-93, teams of Australian engineers taught Afghan locals landmine awareness and clearing. More recently, Australians mentored units of the Afghan National Army, a mission which continues.

Following the 1999 intervention in East Timor, Australians instructed the new nation's military as it transitioned from a guerilla force.

Up to 2008, the Australian Army Training Team-Iraq instructed in counter insurgency, weapons and machinery maintenance and even bread making.

Typically, the Aussies train and then head home, with little visibility of just how their trainees perform once they're on their own. Occasionally the results surprise.

Towards the end of the Vietnam war, Australian and US forces concentrated on improving South Vietnamese forces so they could stand without outside help.

These units were seldom highly regarded and among the more dismal was the 18th Infantry Division.

Australian soldiers had worked hard to lift the standard of some of its battalions, with seemingly little to show for their efforts.

Yet at Xuan Loc, the final apocalyptic battle of the Vietnam war in April 1975, the 18th shone, fighting with a tenacity which stunned their far more numerous North Vietnamese opponents.

They held out for two weeks, and many regard this as the best performance of any South Vietnamese unit of the entire war.