It's not the first time an Xbox One has been available for this price in Australia, but this time it seems to be a JB-specific promotion.
The console has been available in it's basic form for a while, but this is the first time we've seen JB Hi-Fi lower the price of the console outside of a special promotion. For $399, you'll be able to get the 500GB model of the XB1 along with a 1 month subscription to Xbox Live Gold, a wireless controller and the headset.
Other retailers don't seem to have jumped on board. Dick Smith is still selling the basic XB1 bundle for $499, while EB Games is charging $418 if you just want the pre-owned console. Better deals are available -- NBA 2k16 and FIFA 16 along with a 500GB XB1 for $499 isn't too bad.
But if you want the console for as cheap as humanly possible, without going to Gumtree or some other second-hand route, JB's deal is the best you'll get. Target's eBay store is still charging $599 for a new 500GB XB1, and Big W's bundles cost exactly the same as the base console itself.