A couple of points to consider here:
1. CA/GRS Units do not receive any flight plans, nor do they receive handoff coordination from Centre on IFR flights. They have to build the traffic picture from the first radio contact with the pilot and then determine who gets what traffic and when. From a safety perspective, it is obvious that the earlier this contact occurs the better the situational awareness the CA/GRO will have.
2. CA/GROs are not always in a position to monitor the Centre VHF frequency, nor is their a requirement to do so. Some CA/GROs do and some don't. Sometimes the CTAF radio traffic is so busy that the CA/GRO has to turn the Centre frequency down. Therefore, It cannot be assumed that the CA/GRO has the latest information on any particular flight.
3. My understanding is the 150nm calls are being made under agreement between Qantas and the CA/GRS Units in order to improve the CA/GROs situational awareness as discussed under point 1.
4. In my experience, all QF pilots make this call. I can't comment on the other airlines.