right now in the day its going to be in between 12 - 17.
With the wind chill sometimes it can fell colder.
Prolly going to rain at some point, not every day though, and its not all day so just remember to bring an umbrella and you'll be fine.
thing about australia is if the sun comes out its going to feel warm,
so layers is the go.
pants/jeans, t-shirt, warm jacket and another jacket on top if your the type of person who gets cold. this way you can adjust to the unpredictable melbourne weather. and trust me it is :)
At night its going to be colder, anywhere between 0 - 12. bit chilly but not that bad really.
Have a great time!!!