Where does your gf live..? With you..? Or at another residence. If she does not live with you then get a copy of her address, the blue book and her ID card and say you live with her. Or go to BKK as discussed above....Yes TM30 is a real pain...
Oh right. Yes there should be no reason why you can't leave Thailand on a Thai passport and enter Australia on an UK passport with an E Visa and return the way you said....back to Thailand....as far as I know anyway.