Exit Thailand on the Thai passport, enter Australia on the Australian. Exit Australia on the Australian and enter Thailand back on the Thai passport. Pretty simple, done it many times with my Aus/Thai kids. If you are travelling with the mum, then no problem or need for Ampur stuff. Lots of conflicting info re; need for permission from mum letters. Personally, I have taken my son to Aust without it - and had no problems. If you are worried about it, then cover your bases with a visit to the amphur.