When you check in with TG in Sydney, bring your e-ticket numbers and record locator receipts for the other trip and ask them to check you through to HKG on the C flight.

If they do that, and they check your bags through as well, you can just use the transfer security. If not, you would have to enter Thailand, collect your bags, go back up to checkin with CX ...

The Cathay Pacific lounge will be much closer to where their flights leave from and they have a noodle bar. I'm not sure if you would be able to use the TG F lounge or not, but it's really far from your gate and I don't believe it's an arrivals lounge.

-David