Detect maybe, Scanter 6002 isn't up to the task to do air surveillance anyways.
10-15nm and a max of 6000ft is great for helicopter control but not air warfare. It's a radar that doesn't have a 3D capability and certainly can't do volume search. . Shoot down? No. No drone or aircraft worth its salt is going to get anywhere near a 57 mm range. It can watch or kill the OPV from well beyond if it wanted to.
Its a OPV. It's job is to patrol, report, and when it needs to go kinetic its going to be on unarmed trawlers and deal with drug runners. Let the warships and warplanes do the warfare.
Two things here. Who cares if the gun is used by everyone else. That's irrelevant to the capabilities. I wouldn't take Thailand, Malaysia or Burnei's idea of what they need as the same thing as Australia needs. And the US patrol cutters are frigates in anything but name. They usually pack an SPQ-9B fire control radar, Phalanx, 57mm, have been loaded with harpoons, and also have an EWS.
Secondly as I pointed out above the Scanter 6002 is not a Fire Control Radar. EOIR which is what the OPV is going to have will not unlock anything resembling the full capabilities of a 57mm even if you could integrate the two together. Integrating into a 9LV combat system means absolutely nothing if you can't get the targeting solution because your sensors are not good enough.
I can't speak to the threat assessment because there isn't a reference. But I'm willing to bet the RAN have decided that the OPV's will be used in a more permissive threat environment then originally planned and that the RAN and RAAF warships will be used for proper warship jobs.
EDIT: You know last time I looked the RAAF weren't flying warships
Ngatimozart.