As has been mentioned elsewhere on this forum, DFES will be using the WAPol P25 network. They will have their own talk-groups, with probably limited common groups between WAPol and DFES. I'm unsure whether they will be running encryption or not (knowing DFES and it's lust for shiny toys, probably) but if they do it will be using their own key set. Again, there may be some common keys available for interaction between DFES and WAPol, and the system is intelligent enough to negotiate these between disparate radios.
My understanding is that the P25 system will be used for 6AR, and patched to the current analogue 6AR/6IP channels. WAPol have some concerns regarding how this will impact the utilisation of their network; having one big talk-group with many hundreds of subscribers across the network could mean that every transmission on that talk-group lights up nearly every repeater site and ties up a channel. WAPol's talk-groups are generally limited to either geographic areas, or small operational groups (K9, TRG, etc). I am yet to hear whether or not this has been resolved.
There are pros and cons to the P25 system being trialled. In 30 years involvement with fire fighting, the underlying radio technology has not changed (new band, new channels, same tech). Talk-groups are certainly a vast improvement upon the conventional analogue system used by WAERN. No more having to look-up which channel to use when travelling out of your area; just select the 6AR/6IP talk group and talk. The trunking system will seamlessly hand you from repeater to repeater as you travel, without needing any input from the user. Other benefits of P25 include clearer audio and advanced radio features (private calls, status messages, emergency alerts, etc).
Having said that, the greatest and more important improvement ever achieved within WA's fire-fighting radio network was getting all three services (WAFB/BFS/CALM) onto the one radio system. In days gone by local governments operated their own radio systems and attending a fire out of your area could mean not being able to talk to anyone else on the fire ground. The amalgamation of WAFB and BFS into FESA largely solved that and everyone's chief problem then became finding free channels.
This current move to separate FRS from BFS/DPaW, as well as DPaW's experimentation with P25, is deeply concerning and a huge step backwards. Yes, everyone will still have the WAERN radios, but I lack confidence the new radios will always be used competently and appropriately.
Imagine the scenario of a bushfire just starting up. A couple of red-trucks rock up and begin tackling the fire, and because they're new and shiny they use their P25 radios. The fire progresses and more red-trucks rock up. They inevitably lose it and have to call in support from BFS and DPaW. BFS and DPaW don't have P25 radios. Suddenly, at the worst possible time when the fire is really cranking and the IC/IMT are struggling with the chaos that is rapidly arriving crews they now also have to manage re-channelling the fire ground onto new channels that everyone can use. Changing an IC or sector channel is dodgy at the best of times; more than once I've seen it take hours to finally get everyone onto the channels they should be on. Add into the mix a second radio and the outcome is unlikely to be good.
I apologise for such a long post - I had intended to simply set the record straight on the digital radios, but as you can tell this is something I feel very strongly about, and so had a bit of a rant.