Do you mean you have no idea who he is,,,just have a letter and think he is related??
What other KIRBY family do you have in England?
Does a William show with any family during the various census?
To be honest the beginning of your search is in England (not Australia)
You need to find any William KIRBY's that suddenly go missing.
We cannot really do anything without possible parents names,,approx date of birth etc. as there are quite a few William KIRBY..