I was chatting with an AFP officer in MEL recently and asked him the best way to contact them if the need arises and he said if the problem occurs airborne make the request through ATC and the AFP will meet the a/c in the aerobridge or on the ground or both if required.
If you need AFP assistance on the gound then call 131AFP, this call goes to the OPS centre in Canberra, tell them which airport and gate you are at and response will be quick smart.
Info from the
Australian Federal Police - AFP website.
Police attendance at Airports/ You require police assistance at InternationalAirports at both Domestic and International terminals/ Ph: 131 AFP (131 237)
Regards,
BH