There really should be a "maybe" option.
If I was chasing the keyword for SEO, then I would go for the .net.au, if I was after a brandable name that wasn't necessarily the main keyword for the site, then no - i'd need the .com.au to sleep at night!
DMP said:I would for SEO if the .com.au was taken but not being used to its full potential, ie. parked
soj said:I would buy the .net.au equivalent if the keywords had a good search volume and i could monetise them. Also DMP makes the good point about if the .com.au was parked, then the .net.au would be the next best thing.