Depends on your culture, the venue and on the time of day. I'm not sure what the usual procedure is in Oz, but here in the UK women are expected to wear some sort of hat or fascinator for a church wedding.
I've found that a smart dress (not too short) or suit is fine. We generally frown on bared shoulders for church weddings, so a jacket or shawl is a good idea - or a smart blouse.
I always found that dressing elegantly, as if you were going to a job interview, will go down well as long as it isn't too dowdy. You can always jazz it up with some accessories.
I'd also recommend wearing smart heels to the wedding, and bringing a set of flats with you for the reception. Wearing heels for hours on end can be painful, especially when there's dancing involved.
Heck, as long as you don't turn up in the altogether and you remember to bring a prezzie, I'm sure you'll do fine.