it will not 'see' every bit of low level Oz airspace (low level Oz -thats where Ag planes live - below 1,000 feet)
True... had the crop-duster been Mode C transponder-equipped, I probably would not have seen it on RADAR at low-level... due to the RADAR shadow.
The ADS-B coverage at low-level will be much better than the RADAR coverage that currently exists overhead that airstrip for two reasons:
The RADAR beam points up and has a large shadow beneath the beam. Anyone who has called ATC and received the response... "not yet identified" would understand that they are in VHF coverage, but not in RADAR coverage. You can be in VHF coverage but not in RADAR coverage.
VHF coverage is a good example of the proposed ADS-B coverage. The VHF coverage at low level is superior to the RADAR coverage where the VHF site and RADAR site are co-located... or sited relatively close together. The additional sites planned for WA, not including Kalamunda, will provide good ADS-B coverage over the airstrip in question.
I could talk with the pilot of the RFDS aircraft on VHF, therefore, in all likelihood, both aircraft would have been in ADS-B coverage and displayed on my screen.
Therefore, ADS-B would have prevented this Close Proximity whereas SSR RADAR interrogating the Mode C equipped crop-duster would not have prevented it.