Welcome to AFF
@Cathqld .
Is this a paid or points booking?
Can you give us some additional detail? What is the new flight/timing for SYD-DFW and have they given you any new flight BNE-SYD?
Often they do give you unworkable alternatives - you have to call. Unless you have high status with QFF, you'll get an offshore call centre. If the flights you have are unworkable, DON'T ACCEPT any on-line suggestions. Research flights that are workable for you (make some dummy bookings), then insist that these be the replacement flights. If going via LAX, be aware that you'll need to clear immigration and customs on arrival ("enter the USA"), then go back through security, so you'll need at least 4 hours connection time to allow for arrival delays. If the onward flight is on the same ticket (it will be), you will be looked after if you miss the onward flight, but it may be on a much later flight.
This assumes its a paid flight. If its points, its more difficult.
The offshore call centres are often clueless; they may just say "a refund or nothing". If you can't make headway, hang up and call again.