We're an hour north of Sydney and it doesn't get that cold here in winter. I'm usually in jeans and a thin cardi in winter.
The weather will be warmer in Queensland, Northern Territory and Western Australia.
Queensland is great for beaches and theme parks (Movie World, Sea World, Dreamworld, Wet n Wild, White Water World. Our first Lego store has just opened at Dreamworld) North Queensland had the Great Barrier Reef etc
Western Australia is stunning, there's the Pinnacle's, Rottnest Island, Margaret River, Perth, then up north is Ningaloo Reef where you can swim with whale sharks.
Sydney has Taronga Zoo, Opera House, Harbour Bridge (you can walk across it or do a bridge climb) Luna Park, Botanic Gardens, shopping etc.
Canberra will be the coldest, they have Parliament House, War Memorial, Royal Mint, Questacon (fab for little kids!) Australian Institute of Sport.
Melbourne will be quite cold, they have the zoo, shopping, museums, aquarium. There's a new Lego Discovery Centre opening in Melbourne soon. We love Melbourne!