Don't bother trying to cover the old slab, just rip it now and save the heartache later. The way the U.S. guys reinforce concrete is very different to Aus. so stick with your local regs.
Do you plan to build it yourself? not hard with planning and patience. Don't scrimp on fill or compaction. Try to avoid recycled fill, the stuff I got didn't seem to compact well and has dried and hollowed out in places. Evident by drumming under the bathroom floor.
Consider future services like data conduits, gas line, sewer and rain water connections. I joined each side roof gutter by a 90mm under the slab and then continuing to the house tank. I laid out an en suite sized bathroom in a corner when pouring the slab with the 100mm run to 1m out of the slab for later.
I don't know if this is info you need, might give you food for thought.
What part of Tassie are you in? I do love it down there, spent a very fun 3 weeks in our MX5 a few years ago. Hope to get back down in a while, so much left to see and things to enjoy again.