In such scenarios where the ordinary constabulary duties have gotten to the point where it has become reasonable to think a hostile, armed response is possible or even likely, then the role is no longer really a constabulary one and therefore should be handled by a proper warship.
After all, in the event that a hostile power has become willing to target or engage an Australian patrol vessel, the hostile platform(s) could be surface warships, subs, or aerial threats. This in turn means that future Australian patrolling assets would need some sort of sensing and awareness capability covering air/surface and subsurface approaches, as well as some ability to respond to threats coming from same, assuming such a scenario does develop.
This necessity to cover air, surface and subsurface would effectively require at a minimum, corvette-level capabilities and IMO frigate systems would likely be more appropriate for Australia. In effect, such a scenario would mean Australia would need to task GP frigates to conduct patrols due to the breadth of potential threats. An upgraded patrol vessel kitted out with some sort of missile/gun CIWS would just not be adequate, since the Australian vessel could be targeted with hostile torpedoes, AShM (air, surface, and/or sub-surfaced launched), hostile air or ASuW from hostile escorts. Once one has started talking about scenarios where an Australian vessel might become the target of maritime strikes launched using anti-ship cruise missile-armed H-6 bombers, then one has gotten into something where upgrades just would not be sufficient. Without redesigning and then rebuilding the vessels, the damage control and redundancies found in a proper warship design just cannot be achieved in the OPV, and if we have gotten to a scenario where the OPV might be threatened by medium-range assets, one has to assume there is a risk of leakers and thus a need for damage control and survivability measures.
