Highest dosage I have personally seen was brewed by an individual and it was 200mg/ml and I was told firsthand that it was a very uncomfortable test (and I'm being kind in my use of terms here).
Now you're talking about a brew that is 50mg higher than the highest I have ever seen. And don't forget that prop is notorious for being a painful test in general. I have personally not found this to be the case, but then I only use gear I brew myself, and even then I keep my prop to 100mg/ml, so make of that what you will.
You brought up another good point. If most people are ignorant, and most are, a dealer could just say that his prop is 250mg/ml (especially in Oz, yes I am WELL aware of the customs situation you guys unfortunately have to deal with). Who is going to be the wiser or say a damned thing when this dealer is probably the only one on the block. Most guys who don't homebrew don't realize that it is not nearly so simple as just throwing enough powder into some oil, BA and BB and achieving a lovely 250mg/ml mix. Maybe with certain esters, but things start to get more complicated the shorter your ester gets.
If your dealer is selling enanthate (likely dosed at somewhere around 250mg/ml) then, if he is the only source you have, I would try that over his 250mg/ml "prop," which is likely, IME, to be either the exact same thing, or to cripple you.
Again, I really do feel for you. I would never, ever move to Australia, and I'd probably move to another country if I did live there. I have met plenty of cool aussies, but you guys' government and the laws it creates regarding gun control and AAS and a few other things I just could never, ever tolerate.