In Qld it looks like the size of bike is irrelevant, it's just single or dual seat. I was half-thinking about getting a 125 scooter to get in and out of town, but without changing it to a single seat rego would be the full $406. Third Party Property insurance was also pretty much the same as the ST.
Seems crazy to me, but then I'm still used to the UK system where capacity affects the road tax and insurance group affects the insurance. Still, seems pretty pointless to get a scooter which is a shame.