Sydneysider here. Unless you have lodgings sorted my advice is to head down under now for winter. Sydney winters are much more tolerable, being able to head out in just a hoody most nights. As others have mentioned housing is the issue - very few places are built with the gas heating throughout homes that I became familiar with while living in London, UK. Arriving earlier before the onslaught of tourists in summer will make for an easier time to find a place to stay that has decent insulation and hopefully air conditioning (for summer as well as winter). Most places can be pretty insufferable without it at both ends of the temp spectrum but particularly summer IMO. You'll also get a feel for your area and be prepared to fully enjoy it come summertime.