Do you have John's 1st marriage certificate,
Reg #: 5693: Florence Maria PATTIE married John MCCARTHY District: Narrandera
As he is the one who would have provided the information on his marriage certificate it would be far more accurate than whoever provide information on his death certificate.
Death certificate has discrepency - that he was 18 when he married first time and that his death age was was 58 this would mean he was born in abt 1868 - and yet his first marriage was in 1882 and if he was 18 would make his birth year abt 1864.
My suggestion would be to get marriage certificate.