Call me radical - but what do people think about a trip to Williamstown in the second half of September then? There are some fab things to photograph there, especially with that giant partially-constructed warship there at the moment. Also, lots of cute Victorian architecture, grand seascapes and city skyline views, HMS Castlemaine, etc etc. Plus proximity to coffee AND fish and chips... that is all :-)
Urban, easy to get to, far more interesting than the botanical gardens.
What about this: Tall Ship Festival in Williamstown from 6-15 September. I went there 3 years ago in the early morning and got some stunning shots before the tourists arrived. Here is the web site: http://www.melbournetallships2013.yachting.org.au/. I will be there anyway every day. Best would be to catch the tall ships arriving in the port or departing from the port, which would suggest meeting on either Friday, 6 September, or Sunday, 15 September. Maybe the 15th suits all of you voting for the first or second half of September, respectively. Don't forget your wide angle lens for the tall ships at the pier. I mostly used a 45mm lens for 6x7.
Haha, Marc, that's really a coincidence. I did not read your post before I posted my suggestion. As an additional bonus, the whole Sea Shepard fleet is currently docked at the Williamstown pier. Not sure if they are still there in September. The war ship Marc is talking about is the HMAS Canberra, the largest ship of the Australian navy. It will be there until February 2014.