In otherwords if they see a couple of shady looking fellows in a conspicuous area.
Airside or landside? The above description fits many spotters themselves - what exactly are the AFP supposed to do when they receive this call? "I've seen two guys taking photos near the fence - they look dodgy". No offence detected, no reasonable belief that any offence will be committed, no reason to search or arrest, unless they're taking photos of things like refuelling equipment and security gates etc. - but that's why everybody with an ASIC has a responsibility to observe and report this stuff - and we have routine patrols to look out for it.
If they're airside - well, if they have no ASIC they'll be arrested anyway (I'm talking major airports, not some small GA apron), and if they do - you've described 70% of baggage handlers and ground staff I know.