I'd be getting on with it - availability on some flights down under already seems quite limited (I've been investigating something similar for myself). I wouldn't expect prices to go down much - more likely is that they'll sell out.
Be sure to check out specialists like AusTravel and Travelbag, they often have unique flight offers via partners like Emirates or Singapore Airlines. I haven't failed to find a bargain yet and it's pretty much always the case that the best deals are booked sooner, not later. Prices will rise as flights fill up.