Hi Helen, and welcome to Rootschat!
If the age given on the birth certificate and death certificate don't line up, that's not unusual.
Do you have the full death certificate? (There should be quite a lot of info on a WA certificate, I think).
Who was his wife? Was she born in Australia?
The next obvious bit of info to track down would be his marriage, if one exists. Because the information there should be given by him directly, it might be more reliable than that on the death certificate.
I don't see an obvious entry for him in WWI in Australia, so perhaps he was in UK forces and came over after the war?