If you're keen to hear aviation traffic, get one that does 108-136mhz. If you're interested in hearing military aviation, get one that does 225-400mhz as well. Radios that do this UHF military aviation band (generally called 'continuous coverage) are not quite as common and, not surprisingly, more expensive.
The chopper communicates with VKI just like any other police vehicle - 131.6 is the police air wing's 'company' frequency for communicating between aircraft and their base during search operations and the like.
POL AIR 61 like any car when moving from suburb to suburb changes channel as it is moving through, so yes you will hear him on the channel that he is flying over.
I mightn't have explained it well enough: polair is tasked by VKI and takes part in police operations on the normal police channels. The airband frequency (131.6) is for communications between police aircraft and the air wing base, typically in relation to the operation of the aircraft.
Polair 61 was very very active at jandakot Airport - while i was flying circuits Polair was conducting winching training in the middle of the designated training area...
They were practicing fast approaches and winching a rescue guy from the chopper to the ground. .....so they were def using 131.6
Found out last night that the WA Police are getting a new Pilatus PC 12 single engine aircraft. They could use it for taking a TRG squad to Kalgoorlie in it if the need arises and be a lot quicker than driving a bunch of vehicles.
The police service have had a variety of fixed-wing planes over the years: single and twin-engine Cessnas, a Beech Baron that was secured as a 'proceeds of crime' a Piper Chieftan, even a Partenavia Observer.
Typically if the TRG was needed in a regional centre, they'd charter something like a King Air: fast and fairly spacious.
The NT police service already operate a PC-12 and they're also used by the RFDS.