@Karl ********
if you want to apply using the e-visa system in Australia you need to be physically in Australia. If you want to apply in one of the Thai embassies in a neighboring country, you physically go to that embassy as they do not use e-visa as of yet.
When you go on the e-visa site it asks you where you are "located" and what consulate/embassy you want to use. If you say located in Vietnam and want to use the Australian embassy it will tell you that you are not able to use the system. If you say you are in Australia when you aren't, that violates the terms and conditions of the e-visa and could result in asking for entry/exit stamps, a denial or issues when you try to enter.