If you are downing unmanned platforms like drones, is that lethal force? Or if you evacuate the crew before firing and sinking. But it is an interesting point.Border Force is not authorised to use Lethal Force above small arms. Even 5O cal could be problematical from a legal perspective..
TBH the Arafura's could probably do their job pretty well with Typhoons. I am not sure procurement of a specific policing round is really required for these ships. If they face anything where a 25mm is more than overkill, then you need a more capable ship than a Arafura.The Typhoons could be passed down to the Constabulary vessels.
Navy and Border Force.
Part of the issue is the drones aren't actually targeting military ships all the time. They know a US destroyer can easily fend off an attack from a drone travelling at 200kmph. But the large tanker ~40-50km away can't do anything, and at that range, its probably even beyond ESSM from that ship.I think with that slow speed the 127mm(on the Anzac) is already a good solution to take down 5-6 before they reach CIWS range. If Leonardo develops a DART/STRALES for the 127mm probably BAE can sell it as it did with the Vulcano. That in combination with the ESSM should be more than enough.
If the drones are flying at the Navy ships, ESSM and gun based solutions are very viable. 5" is probably a good powerful calibre for larger and high flying drones. I think the 57 and 76mm are quite nice ballistically and cost wise and ships would be able to carry large number of rounds, but engage drones at still considerable range from the ship.
An armed flying UAV might also be useful. Able to take out drones with missiles or perhaps, gunfire. Being airborne and moving at speed, it would have a significantly larger engagement envelope. I am unaware of any existing UAV's with this specific mission.