Since Telefónica, O2 and giffgaff's parent company, do not have a presence in Australia I'd guess, even if there was a will here to expand internationally, it would be very difficult to do especially in regard to initial funding.
If you are serious about this, I'd start by polling the MVNO friendly MNOs in Australia and see if any of them show any interest in running a wholly owned subsidiary network run on a similar crowd sourced model to giffgaff. Optus and Telstra might be a good place to start.
On the other hand, you may have access to a few million dollars yourself in which case, that's bonza mate, go for it! 😉