I believe they take a snapshot (which can be as simple as fork(), hooray for COW memory) of the array every minute to see how much you've downloaded.
The same IPv4 address isn't reused by another customer until a day has passed.
>Most residential customers don't have static IP addresses
Where is this? What do you have to back up this claim?
>how does that work when a customer's IPv4 address changes?
As far as I can tell, it wouldn't. But I don't see what your point is.
I'm not in Australia, but I've had the same IP for decades, even having gone through multiple routers, so it's probably tied to the DSLAM port I'm connected to, which makes sense for such a billing scheme. Also might be why I don't have IPv6 yet either.