"...Thomas states that he is from Launceston on his 5th child's birth certificate (in 1898 Victoria)..."
Can you confirm please, that the name of the informant on this certificate is Thomas Smith?
Is Thomas older or younger that his brother John, and what are their death years?
It is possible that John and Thomas have same father but differnet mothers, or same mother but different fathers, with one brother taking name of stepfather etc. etc. Lots of variations in family structure in era when people died young.