Yep, as a child I lived with my grandmother in Karragullen from 1969 to 1974 and I continued to visit her after I moved to Perth, through to the 1990's. Lots of bush tracks all around that area (Carmel, Pickering Brook, Kalamunda) and although some tracks had signs at the main roads saying entry was prohibited due to the risk of dieback disease, the bush tracks didn't have any kind of gates or fencing in those days as i recall, to stop ordinary people from driving down them.
Most city people though, hadn't even heard of Karragullen up until the '90's though...not until those bodies that you mentioned were discovered, anyway...so whether it was known to CSK is a question. If he did know of it then yes, it would make a perfect dumping ground.