Here's the situation: I am flying with my wife and daughter on separate reservations from HKT-HKG in economy. My wife and daughter will continue without me HKG-SYD in Premium Economy on a separate ticket (I travel separately onward to the US, not on CX). They have three bags to check through to SYD. Will they be allowed to check all 3 combined all the way through to SYD without a fee? Their allowance on that leg is 2x2=4 bags. However, the allowance is 1x2=2 bags on the HKT-HKG leg. Worst case, I can check their third bag, and we can clear customs in HKG and recheck that one separately for them. However, obviously it would be preferable to just check through and pay no fee and avoid immigration and customs. Neither of them have any oneworld status. The layover in HKG is 7 hours if that is relevant. Thank you!

You will need to be prepared for that. I believe this will very likely be what you would need to do if you do not want to pay the extra bag fee. If there is any leniency from the agent (ex-Thailand it's not uncommon), consider yourself very lucky.

With 7 hours its pretty much a non-issue anyway - with so much time, and as long as it isn't a late night connection, I would suggest you do properly enter Hong Kong and visit a nearby mall (in Tung Chung or the ill-fated 11 skies right next to the airport), or check out if something is happening at the Asia World Expo. If you're on the ~5pm HKT-HKG arrival (going thru to a midnight Sydney flight), with no status, and presuming not on a very tightly constrained budget, I would exit the airport and get a public bus (non airport) to the Tung Chung cityplaza mall, have a nice Hong Kong dinner there, then come back, probably taking 3-4 hours and stretching your legs. Rather than spending 7 hours with fairly mediocre food in the airport itself.