Private land, also known as Freehold Land, is land held by a person, organisation of government body under freehold title.
Land owned by the National Trust of Australia is private, as it has either been bought or donated to the trust.
Although some private land is open for public use, the owners can and do impose conditions on its use. The National Trust South Australia owns a number of properties and reserves and requires that any geocacher obtain permission from the Natural Heritage Manager before placing a cache in any of their reserves. A list of their properties and reserves is on their web site.
In Victoria, the metropolitan railway stations, although owned by the State Government, are administered by a private company, Connex, who prohibit photography unless the photographer has a permit. If you place a cache that requires photographing anything at a metropolitan railway station, the finders will need permits.
Geoscience Australia has a break up of public and private lands in Australia.