What's the best way to check for tyre tread depth.

To my knowledge, the legal requirement is 1.6mm

Do I need to buy a special tool, or is there a specific Australian coin I can use?

anything above the wear bar should be a safe amount

Where is the wear bar?

If you follow the tread around the tyre you will eventually see a small raised portion in the tread. Thats your wear indicator, when your tyre wears down to become in line with that you're tyres are illegal.

Look in the groove in the middle of the tyre, and follow it around, you cant miss it.

You could do what the above guys have mentioned and check the wear indicator, that would be the easiest way of telling whether your tyres are still okay, but as said once the tread is inline with that indicator on the tyre you'll be needing a new tyre.

I you want to check the depth, as in actual measurement, id go and buy a tread depth gauge from repco, supercheap or whatever. Its a pen like gauge, with a pin/shaft at one end that sits inside the tread and has measurements at the other end obviously to measure the depth. Pretty simple to use and will take you a few seconds to check the tread depth

otherwise put a matchstick in the tyre and mark off the tread depth then measure the marked area. iirc the tread should be at least deep enough to contain the matchhead too.