I am not sure, William John Sitters, born 24 Apr 1873, Kensington & Norwood. Adelaide. South Australia.
His parents were married here in Australia - William & Sarah Jane (nee Fuller) Sitters. Their son William John Sitter's who is possibily the father of Sidney, so his parents I would say were married in South Australia.
As you said Sidney's mother Elizabeth Johns (maiden name - Biggs?)