I think the 500 Hours is required for the ATPL, and given that at least
VB and QF (don't know about J*) need F/O's to have command endorsements I think that means you will need an ATPL prior to joining (or in QF's case prior to upgrade from S/O).
Might be wrong, but that's how I remember it.
For airlines with a High Capacity AOC the requirement under the CAO's is for only 250hrs PIC. This can be broken up further into 100hrs PIC in an actual aircraft. The remaining 150hrs can be done in a simulator using an approved C&T scheme.
So in actual fact 500hrs Multi IFR is not a CAO requirement for High Capacity operaters such as QF, DJ & J*.
Not sure why they make it a requirement.