No great secret, I am fortunate enough to have access to various BDM CD ROM records which helps!!!
The earliest birth record that I can find on the CD for S.A. births is 1828- there does not appear to be any earlier, and none for a John MOON - the earliest being the son of John MOON and Elizabeth STRONG in 1865.
Be in mind that John MOON Senior may not have been born in S.A. or in Australia for that matter!!