More than 20 Australians who fought with Islamic State in Syria before the government took action to ban travel to the region are free to roam the streets in Australia, according to a report.

The Daily Telegraph reports that the jihadists who were already in the conflict zone before new counter terrorism laws were introduced cannot be jailed for taking part in the brutal war.

The newspaper reports that more than 20 Australians who fought with the terror group are back in Australia, with about half escaping any form of charges.

The men are being closely monitored by national security agencies.

In September Australia banned all travel to the Islamic State-controlled areas in Syria and Iraq without a legitimate reason.

Australians who remained in the area after the ban could face charges.

Foreign Minister Julie Bishop said the goverment's strategy was to stop Australians heading overseas to fight in the first place.

“If they do become radicalised in Australia and then go overseas and fight with these organisations there is a risk they will come back as battle-hardened experienced terrorists,’’ she said.

“People who have become experienced terrorist fighters overseas do try and come back to their home country and try to carry out terrorist attacks.”

A spokeswoman from Attorney-General George Brandis's office told the Daily Telegraph that anyone fighting with ISIL should leave the conflict zone now.

“Anyone fighting with IS and other terrorists should end their association with these groups now and leave the conflict zone,’’ she said.

“Legislation recently passed by the parliament means those who leave Australia to engage in terrorism can be arrested (and) prosecuted ... on their return.