Just Cause 3 pre-order is available at GreenManGaming, with a base discount of 10% off the standard price ($59.99), then the coupon code gives another 23% off, bringing it down to $41.58 USD (total discount ~31%). For comparison, Steam has the usual 10% discount from $59.99 (which is at least impressive for once having no added Australia tax).
Just Cause 3 doesn't get released until 1 December, so it's a long time to wait for the pre-order, but most demos of the game (including youtubers playing it) seem to indicate it's very good already.
Remember, it's always a risk to pre-order, but if you're planning to do it anyway, I don't think you'll get it much cheaper than this (other than dodgy key re-selling sites).
If you haven't got a GreenManGaming account yet, please consider using my referral code. It will mean we each get $2 credit once you make a purchase: http://www.greenmangaming.com/?gmgr=kucipixo
Edit: thanks to Spackbace, you can find it at OzGameShop for a couple of dollars cheaper: http://www.ozgameshop.com/pc-games/just-cause-3-day-one-edit…. Personally I think I'd rather go with GMG (I've had pre-ordered games from OzGameShop take a little while after release to be sent out), but the option is there.
So that's $55 AUD?
USD$38.94 ($53.64 AUD) for steam version
$52.99 AUD for steam version