Keeping in mind I am talking generally here, and am not a lawyer etc.

You can employ someone as "casual" which means you basically work shift to shift - and generally they only have to give 1 day/shift notice for termination. You don't earn PTO on casual. Usually these employees are paid a better rate. But they have less perks. Casual work is pretty good in certain scenarios, I've done it before.

But to attract good workers, workplaces are usually permanent. There's usually a 6 month probation period - in which the first 6 months they can fire you if they're not happy with your performance etc. After being made permanent, they need a really good reason to fire someone (wilful negligence etc).

TLDR: Labour laws/unions are great.

If a company does the dodgy, they're opening their selves up to litigation.

I'm not saying it doesn't happen in Australia, but I would imagine it happens a lot less than the US etc.