Buy soon
Been there once when I was pretty young. Awesome place.
My free flying benefits are about to expire, so I am gonna take a trip around the globe and I want to make Australia one of my stops. I have been checking the loads on the SFO-SYD and LAX-SYD flights and they are incredibly full. My advice is to buy your plane ticket as soon as possible to get a decent fare. United and Quantas fly non-stop from the west coast. Good connections on Air New Zealand and Air Canada. Your looking at about $1,000 round trip.
I am sure you know this, but don't forget to go during THEIR summertime(Dec, Jan), not our summer.