For the domestic sector on TG, please note seat selection cannot be made until OLCI opens 24 hrs prior to departure. This quirk, is probably due to the many varieties of aircraft used on this busy sector.
You will be able to make seat selections for the BKK-MEL flight as soon as you you have the TG PNR. Once you have pulled up the booking under 'manage my Booking', the seat selection process is a bit obscure - scroll to the bottom of the page and select 'additional services', which will bring up seat selection and special meals - easy from there.