Dick - I believe it was put in as an interim measure when Port Hedland was supposed to become a Tower. The traffic disappeared with the end of the mining boom and the interim AFIS has operated ever since staffed by ATCs from Karratha. The broadcast area is the same size as the Ayers Rock broadcast area.
FAA Class C can be 5nm because it is surrounded by Class E which contains the instrument approaches and missed approaches. Our Class D and C is bigger to do the same job becuase of the lack of Class E.