Hi Steve
I do not believe that Harley is connected to the postal service, but that his address is simply the Post Office in Kokstad, a small town ,and a common way of addressing post in remote areas at the time.
Mervyn Frederick Walter Harley married Emma Phyllis Strachan (born in Flagstaff, died 1967) in Kokstad in 1913, and he died on 20 April 1920.
The Conductor rank was that of a special class of NCO in a Commissariat and Transport Department.
Regards
Adrian