Check out the following website for general information and on that website note the link to the Victorian Friends Network Website which will provide you with more specific details.
Parks Victoria: Friends and Volunteers Groups page
I guess it will depend on where you live in the Melbourne metro area but any of the many Friends Conservation Groups would welcome you with open arms.
Another good source of obtaining information on volunteering in a Friends Group would be your local council.
I hope my information is of assistance.