Would you say in your humble opinion there is much value to this domain extension? I checked Namebio and there seems to be some pretty active sales so I assume they must hold some value?
Probably it depends mostly on what stands before .com.au.
There should be no trouble selling one-word premium with that extension. Like security(.)com.au.
Well, i believe you should not be worried if domains you are looking at make sense.
I don't have much expirience or knowledge to share, but i will keep in mind "Quality over quantity".