The Australian Tax File Number is presumably more similar to the Social Security ID? Millions of Australians don't have a passport. You don't need one for much - it's perhaps the easiest way of verifying citizenship if you already have one but not the only way.

You would only have a TFN if you are working and potentially paying tax. So generally anyone under 16 would not have one.

Closest might be a Medicare Card which gives you access to free/discounted public health that can be used as part of identification. Usually children are on their parents card.

Drivers licence is also a primary identifier, and students can use their school student id.

In Australia you start with a birth certificate and photo, and that leads to passport and driving licence. The three of those are the holy trinity of ID (though you'd very rarely be asked for your birth certificate).

With passport and driving licence, you can do anything you want, but at least they are photo ID with some anti-forgery features.

The time to steal someone's identity is before they get their first driver's licence and passport!