I cannot see a birth for a David McMahon in 1867 in SA either 
I rarely place much reliance on submitted trees, but may I note that there is a submitted tree which has David as born in IRELAND.
The tree also has uploaded the 1899 marriage cert for David McMAHON.
12 December 1899 married in the District of Adelaide.
David McMAHON 32, Bachelor, (Trade or Calling) Engineer, (Residence at time of Marriage) Adelaide, (Name and Surname of Father) James McMahon,
(Place in which celebrated) : Rev J Thome's House, Franklin St, Adelaide
Emma Lilian DANIEL 22, Spinster, Adelaide, (Father:) James Daniel.
Witnesses John (cannot clearly decipher surname, could be Minapav) Bootmaker, Franklin St, Adelaide and Elizabeth Jane LAYTON, Charwoman of O'Halloran St, Adelaide.
Bride, Groom and Witnesses all signed. So too John Thome, Officiating Minister.
Cheers, JM