Thanks, kitnut!
I was thinking the 1980-to-2000 period, after the RAN FAA de-commissioned its Trackers in my time-line (as opposed to RW, when they were de-commissioned in 1984 & pretty much left to rot).
That makes sense. Maybe have the story being that after the RAN retires them, the aircraft are acquired/provided to someone such as Surveillance Australia (more on this below) to form the Coastwatch style organisation a few years earlier than in the real world.
The Trackers will be used purely for the patrol/surveillance role & have no weapons or ASW-specific equipment fitted; think Customs, Immigration, Fisheries, Shipping Control & EPA as the primary functions, with some ability to carry out surveillance of foreign warships & military aircraft which come too close to Australian waters/airspace.
So, something akin to the real world Surveillance Australia/Coastwatch program. In this case, you can so away with a lot of equipment and simplify things. I would still give them a new radar and EO system, probably similar to that used by the Dash 8 aircraft - see here. You could also use the Australian Customs IAI 124N Sea Scan aircraft as a point of inspiration.
In my time line (the same one for the RAM & the Grumman Grebe,
et al.) the RAN Coast Guard is formed shortly post-WW1 with a few CMB's & some fast ex-pearling luggers as a counter to smuggling activities across the Top End. A lack of dedicated funds & some questionable activities saw it fold in the mid-30's. However, immediately post-WW2 a few Dog Boats (Fairmile D's) & a couple of corvettes (
Bathurst's), plus a small number of Catalinas were allocated for training & small-scale operations to be the seed of a larger & more professional RAN CG covering both RAN & civilian agency patrol & surveillance requirements.
The Trackers are merely transferred directly to the Coast Guard, rather than sold (the CG operating on a mix of hand-me-downs & new, purpose built equipment).
The Dash-8's look like a good starting point for base-line surveillance systems with some ELTA lumps & bumps added for improved ELINT. Not sure how the SeaScan nose would work on a Tracker but maybe something similar added low & just aft of the cockpit on an extended forward fuselage might work.
The technical details may, of course, be a little bit ... um .. fudged to suit whiffability &, to some degree, the Rule of Cool.
Thanks for the ideas!