Unless you're willing to leave your house sitting empty I wouldn't book until the house is sold. It does cost more at that time of year to fly to Australia but it costs that much whether you buy now or late. It doesnt always follow that the prices will be more if you do a late booking. If there are empty seats they often sell the off to get bums in them! That is unless you want to fly over specific holidays like christmas and easter where the prices soar on late bookings! And if you want fully flexible tickets that you can change the date on in case the house is not sold you would pay more again.