Illegal in most Australian States and Territories except Western Australia. Here in NSW the fine exceeds $1,000 for simply having it the car, switched off on the floor!
Police can, and will detect 99% of these devices, so be careful.
I personally see no reason why they are prohibited, aside from politically correct posturings a few years ago when the ARR's were developed. Their prohibition was not the basis that cars using them were wiping themselves and other drivers out, that much is certain. Also accepting the fact that 80% of speed related crashes occure below existing speed limits in any case!
IF you WA folk et al use one, just don't drive like a tool or at lunatic speeds.
WA would be wise to NOT ban the devices, ways and means to set them off to slow down or 'alert' the <1% of drivers that use them.