Melbourne has many rooftop bars, but only a few are accessible of course.
Firstly, here is a site that has info on bars:
Unfortunately, on this site you have to look at “features” under each bar entry for “disability access”.
The search on this site has all sorts of options, such as “wheelchair access”, “location”, reviews and so on.
This is a new site (2017). If you hover over the little building symbol on the top right and select an option you get a whole lot of filters on the left. Right near the bottom you can select wheelchair access.
Here are some that I know of:
The Corner Hotel rooftop bar
I have seen some great acts in this pub in Richmond. They have a dodgy makeshift ramp they put out for wheelies although there have just undergone a renovation so they might have made it a bit easier. I know they put a lift in so wheelies can get to the new rooftop bar.