Hi Faye,
So I suppose you've been through all this but just in case - did James get married? If so that cert should have more info? Also, did he have children? If so would their names give you any clues? Do you know what James worked as is Dubbo? Do you think there might be a write up in the local paper when he died? Was there a will or probate for his estate - the executors might give you a clue?
Also, where did the death cert say he was born?
Hope this might help,
Naomi