... think about it mate! The areas that need the PRIM eyes to see sneekies will still have it, and where it won't exist, it currently does not anyway!
Bing, listen to Scurvy. Primary Radar is the one that bounces a beam off anything metallic that's moving... including, on occasion, trucks barreling down the highway. Primary Radar exists in all of the big cities... and it's not about to be de-commissioned.
Secondary Surveillance Radar - SSR, over a broader area of the East Coast, plus Perth, Adelaide and Darwin, is the RADAR that is proposed to be replaced by ADS-B. SSR is not a surveillance RADAR used by Defence because it is a participatory system of surveillance, that is, you have to agree to participate in the system for the the surveillance to register your presence. If you don't turn on your transponder, we can't see you.
ADS-B is also a participatory system of surveillance, but a far more advanced system... more accurate, more reliable, more data transferred (not just a selectable four-digit code and pressure altitude), and a lot more cheaper. Because it's cheaper, we can justify installing it across the continent, and thereby achieve a RADAR-like system for the whole country, not just a small corridor around the East and Southern coastlines plus a small circle around Perth and Darwin... instead, the whole continent.
If your argument is that SSR should be retained for security reasons... think again. It is total useless as a form of surveillance for Defence purposes. But then, there are far, far better means of surveillance in use than RADAR. It has had it's day, a very significant day, but it is no longer the means of surveillance of choice, it is primitive and antiquated when compared with other technologies and therefore... insignificant in the security world.