The 23-year-old defender had signed contract acceptance forms with the Saddlers, indicating he was set to remain at the Banks's Stadium.
However, after discussions with boss Chris Hutchings he has decided to move Down Under.
"The offer came a little bit out of the blue," said Hughes. "It is a once-in-a-lifetime opportunity and if I don't go for it I think I'll always be thinking 'what if'.
"It's not a decision I've taken lightly. I've really enjoyed my time at Walsall and will miss the players, staff and fans. It's a good club and I wish them well."
Hutchings explained: "We spoke at length about the move and agreed to go our separate ways. Mark feels that this is an opportunity he wants to pursue and we will not stand in his way."