I agree with Chrisell that Callan's popularity in Queensland probably comes from its similarity in sound to
Callum. But it may not be a complete creation, because Callan and Callen exist as Scottish and Irish surnames. According to Reaney &
Wilson's
A Dictionary of English Surnames, in Scotland Callan is a variation of MacAllan, and in
Ireland it is derived from Cathalan, a diminutive of a name meaning "battle mighty."