So it sounds like you have decided it is best for both of you to clear Immigration in one of the two main arriving Immigration Halls at BKK, claim any checked bags, clear Customs, head up to level 4 to check-in with TG, then maybe meet land-side? After which you will clear domestic departures security to access the domestic departures air-side area?
Or are you going to check your bags through to CNX - this should be possible at both LHR and SYD - clear mid-field Immigration at BKK, get boarding cards for BKK-CNX at the transfer counter, then meet in the domestic departures air-side area?
Both processes are detailed in the links I provided.
Land-side there are restaurants and coffee shops on level 3, and Level B (airport rail-link), while there is a prepaid food court on level 1 near door #10.