~> Sharing the information, as it has changed slightly recently, and from what I researched, isn't readily found on ParksVic website...
Seasonal roads, eg the dirt section of road up to The Horn; and also the dirt road leading out to The Reservoir (ie the access road to climbing areas of Devils Couch / Og Gog Magog, etc); will close on the Thursday after the Queens Birthday long weekend, and open again on the Thursday before the Melbourne Cup weekend.
~> So for 2012 this means;
Close Thursday 14 June.
Open Thursday 1 November.