Yesterday I went to buy a power drill at Bunnings, and took my dad along, since he knows more about this sort of thing than me. Of course, taking my Dad to Bunnings is a bad idea. He treats hardware shopping much like a stereotypical blonde treats shoe shopping.
So we ended up looking at insulation, and I noticed polystyrene sheets! Now, this was not the cool extruded pink or blue stuff, it was expanded white stuff, but I prodded it with a powerful digit, and it seemed pretty dense. There are a few issues - it only comes in huge bundles which cost $60 each, it only comes in one thickness, and some of it appears to have cuts part way through it to allow it the bend... So I didn't buy any.
But this is a hopeful sign for the future - we can dream of the day when Bunnings stocks polystyrene!
BTW, Bunnings has a new branch in Mentone where the old Nylex plant was. We ended up there after the first branch did not have enough insulation to satisfy Dad. It's so big you can just about see the curvature of the earth when you stand at one end and look down the length of the place.