I just received the FF newsletter so answered this thread much later than anybodyelse.
I only showed my Australian passport to get in and out of Australia immigration customs. From then on, I only used my European passport as you can travel trough Europe without showing it to anybody unless you fly, no customs checks at train stations. Even for your return flight, to get out of Europe, you don't need to show your Aussie passport at all, as one poster had said, showing 2 passports will complicate things more than help, keep it simple. I usually keep my Aussie passport for Asia as we mostly don't need entry visas and never bring my European one to travel this side of the globe...again, keep it simple, if you don't need it, don't bring it, save you from worry against loss.
For tax purpose, you can only get TVA back when it's more than 175 euros, which you claim at your final departure and you must have a non-european residential address. Some big shops in Paris give you TVA back right there at the transaction, that's why it's better to do all your shopping in one shop. They give you some forms which you have to present at CDG airport to be stamped and put in the letter box located right next to it, very handy. Theoretically, you have to show the goods, but we never did, we checked-in beforehand already, and customs are not interested to see them anyway, the queue is so long, they don't have time.
