what phil said, it will classify as an import and need to go over pits to meet our regulations.. will need pollution gear for your exhaust and an imobiliser fitted.. other than that from what you say about your little firecracker should be fine
you will find another gc8 wrx here if you arent too attached and want to start from scratch, unless your cars in amazing condition or you have some freaky fetish obsession about it like the rest of us
EDIT- aaand the blow off valve, in australia we are not alowwed
BOV's that dont plumb back into the exhaust. most aftermarket vent into the atmosphere (sounds cool, pacho pachoooo!) and the govermnent thinks you will kill us all instantly with your unfitlered exhaust gasses