Hi Harlequin
Enjoy your trip Down Under, Sydney is a really great city as is Oz altogether.
Re what to wear on a long haul. Be comfortable so wear loose fitting clothes around the waist, have a cardigan or fleece to put on as the air con is often chilly even with a blanket they provide.
Ankles usually swell on most people so have shoes that will fit once you have to get off at Sydney! I always wear flight sox, they certainly keep swelling down and maybe a protection against DVT , there are mixed messages there these days.
Your skin can feel very dry due to the cabin atmosphere so a moisturiser is a good idea.
Otherwise, enjoy the flight, drink plenty of water, and have a great holiday.