Hi Toothfairy. My brother & sister-in-law live in Sydney. They recommend travelling in March so you're spot on. Xmas is horribly expensive and any time over their summer is too busy cos the kids are on holiday. Stick with your original plans.
I think the prices have probably shot up cos people who have been putting off flying are now all clambering to get on a plane. Once the demand reduces to normal levels the prices should even out. My sister in law usually pays about �600 - �650, but she flies at ridiculous times in the night.
Happy searching