Hi Stu,
The Mount Stromlo Observatory visitors centre is open. More information here:
RSAA.
In an old press release on the web site (november 2004) it says that the first Saturday viewing night was held open to the public. However, elsewhere on the web site it says that Saturday night viewing has been cancelled indefinitely owing to construction work. Your best bet would be to give them a ring as matt suggested.
I haven't been there myself yet but I will be soon.