According to the definitive Australian Macquarie Dictionary, the origin of the word is from Irish tinkers' jargon for fellow. The word is much loved by the Australian people, where its 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.