As already stated above it is most unlikely an A380 will be allocated to BKK - SYD.
Slightly off-topic: Long term, First will only be on the A380 and will disappear when that is gone. The A380 is a great plane but not very profitable (Emirates a possible exception) hence the first one delivered to SQ was broken for spares when the lease expired. Now that Business is so good on the newer planes I have only ever paid for First on one leg, the difference is just not worth the cost. I suspect much of First is mileage redemption or GUP/PUP upgrades and, according to rumour, the numerous ex-directors and family who fly free.