Hi Damo,
While I'm no expert on where they occur in Melbourne, I'm pretty sure they are found in wetlands/damp areas/near ponds, as they mainly hunt frogs and lizards. Best time to see snakes in general (on a sunny day) would be in the morning around 9 or 10ish in my experience.
Good luck, and I'm sure some local members on here could help you out a bit more.
Bredli