Led: Where two airspace types meet, the base level where they meet is classified as the lower airspace type. Which means that where a block of G meets C above, the level where they meet is classified as G. ATC keep a buffer between aircraft in controlled airspace and the lower limit of the airspace step. Aircraft OCTA can operate up to and including the base level as depicted on the map.
For example, if the chart has C LL 5500 (East of Canberra for example), ATC will only drop an aircraft to 6000ft there. An aircraft in the Class G below can operate up to 5500ft without a clearance.
Don't have the specific references with me, but all the info is in the Manual of ATS and the AIP.
Cheers.