My limited knowledge of RF would say that's not a SAPOL equipped vehicle. SAPOL use the Government Radio Network (GRN) which are only require a 40cm antenna for their digital signals. Highway vehicles have HF radios which have the big antennas, but theres no reason to put one on a city vehicle like that, and it doesnt look like a HF antenna. I've seen plain clothed officers in an old holden crusing around TTP before, they normally dont equip the vehicle with a mobile radio, but use their handhelds. And they arn't undercover persuit vehicles, but more like a spotter. My guess is its a CB enthusiest, as someone else suggested. Probably have a scanner in there to pick up a wide spectrum of frequencies hence the need for a long antenna.
The End!