@Colin: bike helmets are allowed under the current regs, as long as they meet AS1698. I use one.
BUT as Josh has said, they are not rated for the same kind of impacts as motorsport helmets. A new helmet for me has been long overdue, I guess that makes me a muppet
I have been trying to track down solid info on what the requirements are changing to. I beleive they are dropping AS1698, and possibly a lot of the other older standards as well, and requiring a HANS device. I'm not going to buy a helmet until I know it will meet the new regs.