It's a pre-departure test, but it's required to ENTER Australia. So if you're entering on the 18th, then there's no requirement to have had the test pre-departure, so it shouldn't be required - or at least, that's my interpretation!

That said, the decision here is going to be made by the check-in staff and if it was me I wouldn't be relying on them to interpret it in any consistent manner... If it was me, I'd be having the test, just incase. Australia still allows a PCR test up to 3 calendar days before departure which is well within the time it takes to get results back from basically all of the 'free' tests available. (I got mine a few weeks ago at Kaiser - took 36 hours)