Melbourne isn't a 'beach' city so for me it's a wonderful visit any time of year although I find summers there a bit too much. I'm from Canada so the winter there would be early spring here, give or take. Melb is a great people watching/ street shooting city with a very cafe feel. So if that kind of vibe appeals to you, it'll be a good time. Not sure what it'd be like in the winter but if you have an opportunity to go to Kangaroo Island in South Australia, GO.