Rules can vary from one National Park to another.
Some allow dispersed camping, some only in designated areas and others no camping at all.
State forests in Vic you can generally camp anywhere (there are guidelines eg. away from picnic areas and roads)
For info on Victorian National Parks, State Parks, Regional Parks, Coastal Parks & Historic Areas (did I forget any)
Downloadable Visitor Guides/Park Notes usually give you useful information and basic map.http://www.parkweb.vic.gov.au/
For information on Victorian State Forestshttp://tinyurl.com/vicstateforests
Some have downloadable "Forest Notes" with info and map