Just writing to advise I am a direct descendant of William AM Cairns, who was my Great Grandfather. William served as police trooper prior to joining the AIF in 1916. Although at the time of enlistment he notes his occupation as shop assistance. On joining the AIF, William and Alice resided in what at the time was Homebush NSW, but has now been rezoned to be part of the suburb of North Strathfield.
As you mention William married Alice Thomson and the couple had one child Alice Phillison Cairns, my Grandmother. Alice Cairns married Norman Jeffery Tritton, my Grandfather, and they had two children Jeffery Tritton and Leone Tritton (my mother).
My mother Leone married Barry Peterson, and they had two children Tracey Peterson (myself) and Kim Peterson my younger sister.
Hope this information helps. Please let me know if I can assist in adding any further background.