Sounds like it was a good idea to give him suggestions!!
He should be able to find something small that isn't ridiculously expensive. I personally don't like big opals anyway. If it's something little you'll probably wear it more often too.
You absolutely *must* make him pack any spare space with Tim Tams. Be absolutely clear on this!
Sadly, the kangaroo purses really do exist and they look about as good as they sound. Happily, Australians don't actually use them. I imagine that the only people buying them are those who will enjoy the horrified response from whoever unwraps them.
With your dad choosing, fashion may be out (otherwise Lorna Jane is a great suggestion for a young gymnast type), so I'll have to agree with the jewellery suggestion. Do you need to specifically say you don't want a hat pin shaped like a koala with an opal in its tummy? Because they're as common as kangaroo scrotum purses in souvenir shops here. On the other hand, tiny opal earings and especially black opals are gorgeous, and a small pair of earrings shouldn't be horribly expensive.
Agree with the timtams...best biscuit ever, and nothing else comes close.
I used to make little "Australian" packages to send to kids overseas, and I usually included things like soaps scented with lemon-scented gum (a very Aussie smelling tree...really lovely scent), timtams, cherry ripe chocolates, calendars or hard-cover books with photos of Australia, or Australian animals...
Ask him to go to the supermarket and they'll be in the chocolate/lolly aisle, probably on a bottom shelf.
They're flat rectangular thin chocolates a bit smaller than a playing card, BUT they come in packs of 6 (I think) and are individually packaged with cartoon pictures of really cute little Australian animals. Yes, they are called Furry Friends. Really suitable to give to little children. Very cheap. We're not talking gourmet chocolate here, just very high cuteness value.
Australian stuff that is amazing:
Tim Tams! Seriously though you haven't lived until you have had a tim tam
Vegemite - though you brits might not like it
Lorna Jane do great half tops and slogan work out singlets (almost my entire gym owns at least one)