Looking for a Half Sister Whom I Have Never Met and May Not Know That We My Brothers Exist

This is a real stab in the dark here, but I am looking for my half sister (whom I have never met).?
Her mothers name I believe is Ruth nee Evans living in South Australia around 1967 when and where my sister was born (give or take a year) our fathers name was Robert Buck. Not sure of any other information and my family can't tell me anything else. I have tried searching on Ancestry but there is no South Australian records.
She may not even know we exist! Her mother was living in Adelaide around this time.