If you don’t have multiple entry and you go to Cambodia without advising immigration first, will they cancel your 12 months visa upon your return in Thailand? Then you will have to go through the process of getting a new visa? How about visa extension?
A reentry permit is 1000 THB. A non O (not non O-A here) retirement visa is 5000 THB. The multiple entry permit is 800 THB. Do the maths as I don’t know how much you paid your initial visa…
I am Canadian, Canada and Thailand have a reciprocal fiscal agreement. Income is taxed only in the country where the pension comes from, for me, in Canada. There is no double taxation. You have to do your homework and find out if there is a similar fiscal agreement between Australia and Thailand.