You cant automatically convert your AUD to BTC by simply sending fiat money it to a bitcoin wallet. A bitcoin wallet good for storing bitcoin only holds your cryptocurrencies, nothing else. You actually have to buy the bitcoins first, just like with other stocks and or assets. That's why we have services like coinbase, kraken, localbitcoins, etc, to make buying easier.
I'm not sure if I would call blockchain.info's wallet "decentralized" though, I honestly wouldn't keep huge numbers of bitcoin there. I'd prefer Electrum. I suggest reading this:
[General] Bitcoin Wallets - Which, what, why?Best of luck.