I'd recommend pre-ordering at Amazon as a backup, because you don't have to pay anything until it is released.

And I don't know what the BigW June Toy Sale "exclusive" is this year. It isn't always the Commander Class toy, as it was the Titan Class toy for a few years first (from Metroplex to Omega Supreme).
Plus... Hasbro Australia would often prevent pre-order listings at other Australian outlets until after BigW has their release, to protect their "exclusive first release" status of the toy. Since Armada Optimus was listed on Amazon Australia today, it makes me think that it won't be this year's Transformers exclusive at BigW.