According to the definitive Australian
Macquarie Dictionary, the origin of the word is from
Irish tinker's jargon for
fellow. The word is much loved by the
Australian people, where it's usage is usually reserved for the expression "good bloke", meaning an
honest and reasonable fellow. One never hears of a "bad bloke" (Aussies would call that fellow a
mean bastard).
PS, a "tinker" is a mender of pots and pans