There shouldn't be a problem EXCEPT make sure they have the right visa to enter Australia with their British passports. You just want to clarify that since they've effectively left the country in CNS, whether they will require a new visa to enter Australia again in SYD.

While Qantas may be a good source for reliable information, they may also want to charge you additional surcharges as your friends will be classified as using SYD as a stop-over, not just transit.