We were probably saying it wrong, but in Northern Victoria we just said Wagga once.

Well, for starters, it’s the reverse 😆

And you’d probably have to ask a linguist, but I would think it’s gotta be related to ‘Wagga’ being two syllables, and ‘Woy’ being one. Not only is ‘Woy Woy’ short enough to begin with, but ‘Woy’ is also probably more prone to mishearing than ‘Wagga’.