Here's a list of dive clubs around Melbourne:
And a recent thread on melbourne dive clubs for a better idea:
Your probably going to need to do a night dive for a good chance at Blue ringed octopus, but you can see them during the day moving around, and if turning over rocks is your kind of thing, you might find one sleeping.
Blairgowrie Peir seems to be the cephlapod capital of Melbourne (and critter capital), so if you put that on your list for a night dive, your near guaranteed to see some type of occy.(or so i hear..never actually night dived there).
If your planning on coming over the Bellarine Peninsula side of the bay, let me know and i can point out some good nudi sites.
sorry for the delay in getting back to you. Will take a look at those sites later when I'm not on my phone. Not sure where I'll be heading yet. I know I'll only have 5 days in Melbourne around the 18th April and no car I don't think so it will all depend on where I can get to. I'm not sure how easy it is to get to Blairgowrie etc so it's something I'll have to look into when I'm there.
Really excited now... Just trying to cut the weight of my case down!!