Although warm water and air temps make for much more enjoyable diving, there is surely much you will miss if you don't also dive in the cold. Drysuits make doing this a much more pleasant experience and I recommend that all divers at least consider trying out one sometime.
What are the features and creatures to see in Port Phillip Bay? Do you have any photos or videos of the area? If so, please feel free to share with us.
Ace
Oh, and welcome to our forum. I am sure I can speak for all of us here in wishing you a warm welcome. We really hope to see you here often.