Delatite Station, Mansfield Victoria.
Here in Melbourne we have gardens which although they date back perhaps 200 years and generally not much further are still part of our history. Many times they are not grand estates but are private properties and those who own them may not even be aware of the historic value of the garden. The responsibility can be daunting but help is available through local historic societies, groups such as Heritage Australia or the government equivalent in your area.
If you have an old property which you believe may be of historic value dont be too daunted by the responsibility because there is advise and indeed monetory assistance available for those who wish to find it. Often the rewards can be worth the research.
Good places to start for assistance in Australia include: http://www.environment.gov.au/heritage/index.html
local Historical societies and the National trust. You can also nominate a garden or property if you feel it should be protected.