PSN (unlike XBL which I assume you're more familiar with from your name) is very regional.
ALL codes are limited to their region of origin, and this usually means country.
If you try to use a PS Plus or PSN Credit card that was bought in Australia on an account that is based elsewhere you will simply get an invalid code error message.
DLC is also region specific on PSN, so you can't mix and match. This means if you import a game you HAVE to buy the DLC from the corresponding version of PSN. If you buy it locally, it won't match.
On the upside, you can have multiple region PSN accounts on your PS3 (just like you can have multiple region XBL accounts on your 360) and almost any game downloaded will play on your system. Again, just like the physical discs. The big exception here is Persona.
There are also region restrictions on video and music (you can only buy games on out-of-region accounts, the other media will come up as not available).
In short, if you want an EU account and live in Aus, you'll need to first pick a country in the EU, create an account there and then fund it with cards from that country.