Self preservation suggests that the passport cancellation program should be based on stopping prospective jihadists returning to Australia with increased skills in doing us harm rather than on trying to save the citizens of Iraq or Syria from suffering at the hands of these deluded individuals. After all the numbers of prospective jihadists departing Australia are not sufficient to materially influence the depredations that ISIS inflicts; reversing that involves a military victory over ISIS.
I therefore propose that Australia enact legislation allowing any citizen currently subject to passport cancellation to depart Australia on a valid Australian passport after they sign a form acknowledging that their Australian Passport will be legally and irrevocably cancelled three days post departure. This allows the prospective jihadist enough time to legally transit to their desired war zone but not to return after a meaningful period of terrorist training.
If international law precludes rendering such individuals Stateless then perhaps the Western nations need to aim to have that law redrafted or alternatively assert their individual sovereign interest over international laws that inadvertently give succour to terrorists hell bent on establishing a global caliphate through indiscriminate murder of our citizens.
Allowing departure facilitates the ability of such individuals to participate in producing propaganda with Western voices but it is hard to see that risk exceeding their remaining here and inflamed by an inability to depart

1. They travel in order to commit murder.
2. A state may only legitimately protect those who asked for its protection.
3. Iraq has asked Australia to protect its citizens.
4. The Kurds in Syria have also asked Australia to protect their people.

Therefore it is OK to take the necessary steps to prevent these murders, including the cancellation of passports.

This proposal, while legitimate, does not prevent the murders, hence it is inferior.

Those who clearly pose a danger within Australia, could be either detained indefinitely as enemy P.O.W's or even killed (by women!) - this is a legitimate case of self-defence. In fact, they could be used as collaterals to prevent the murder of Australians and others by the IS.

The problem of letting them go with their passports is they could still travel on them
even if they are cancelled. Many countries do not refer to data bases at the border.
The way to do it is seize the passport, and issue a travel document to a
particular country. The document becomes useless.
This is what deportees get when deported.

Ollie, I'd prefer to allow them to leave, then cancel their passports, because after all, just the fact that they are even considering leaving to fight for the IS is reason enough to boot them out.

Considering we are not actually "at war" with an independent State it's a bit of a legal problem - but it seems that now you're deemed guilty and have to prove your innocence.

If you're already in a country when war is declared, then what?

What about those many citizens who enlist in the Israeli army, kill Palestinians and then return home?

Not long ago Syria was "our enemy" - now they seem to be an ally. Is Syria a no-go destination or not?

I'm surprised that passports are actually required to enter some of these countries.

Logically it should be up to the countries they are entering or transiting through to stop these people.

A new definition of war is needed.
The old system of large national armies proceeding across countries has ended.
The government could declare war on the Islamic State.
Then anyone here who declares support for or who tries to leave to join
that army of that foreign country, could be interned for the duration.
That the war has been going for 1400 years is immaterial.
This is quite comparable with normal practice in wartime.