I asked OzRunways support. The tl;dr of their reply is the last bold paragraph
The longer version is the following email exchange, which also gives more clues. I asked them the following.
Hi. You know how AvPlan and OzRunways only display traffic from people on the same system? AvPlan says they would trade information but that OzRunways does not want to. Is that true?
The replied the following
Thanks for contacting Support.
OzRunways traffic is our own in-house system that only displays other OzRunways users who are flying, have an internet connection, and have their traffic system switched on. It's a great situational awareness tool but it does have its limitations.
For this reason, we recommend the use of a portable ADS-B in and out device in VFR aircraft. As expected, the Government is investing heavily in ADS-B for VFR aircraft which suggests they are one the same page as us in that ADS-B is the safest option.
https://business.gov.au/grants-and-programs/automatic-dependent-sur...
What we typically recommend is to get a SkyEcho portable ADS-B IN & OUT device. This device is included in the rebate program above.(https://www.ozrunways.com/store/adsb/skyecho.jsp)
Skyecho 2 hooks up to OzRunways, picks up ADS-B OUT signals from other aircraft and displays them just like the regular OzRunways traffic on your map. All IFR aircraft and a growing number of VFR aircraft are ADS-B OUT equipped so you'll notice a receiver like the SkyEcho will allow you to see a lot more traffic.
If your aircraft isn't already ADS-B OUT equipped, the SkyEcho will give you portable ADS-B OUT functionality which means anyone with an ADS-B IN receiver can pick you up too.
The great thing about using an ADS-B receiver is that it doesn't rely on having an internet connection, it picks up signals directly from other aircraft.
Devices capable of portable ADS-B OUT are called Electronic Conspicuity devices and you can read more about that here: https://www.casa.gov.au/operations-safety-and-travel/airspace/commu...
For a bit of a shorter summary check out our article here: https://ozrunways.tenderapp.com/kb/frequently-asked-questions/skyec...
If your aircraft is already ADS-B OUT equipped you could get something like the uAvionix Ping which is an ADS-B IN receiver only, and therefore a bit cheaper. (https://www.ozrunways.com/store/adsb/ping.jsp)
Let us know if you need further assistance.
Regards,
I asked them
Are you not allowed to answer my question? It was as follows....
A different person, maybe a more senior one said the following.
We believe that ADS-B is the way to go. We run and control our own traffic system and will continue to do so at least until ADS-B is ubiquitous. For a host of reasons – reliability and user data privacy being the big ones – we are not looking to make our traffic data available externally.
For all the reasons [redacted by me, not that it's a secret] mentioned above we typically point people in the direction of ADS-B devices, fixed or portable as appropriate.
So, I asked them
How would sharing your data decrease reliability?
"Traffic is one of those things where you want the data to be as up-to-date as possible. Looking at a traffic icon that is 20-30 seconds or more behind its actual location is not particularly useful. Sending and receiving data through more servers etc. introduces more latency and the end result is traffic symbols that have some unknown amount of delay in them.
This is just another reason we recommend ADS-B – signals are transmitted & received directly by the hardware in the aircraft meaning it's about as instantaneous as you can get, and it doesn't rely on any internet or cellular connectivity."
My own view is that knowing where someone was a mile ago is better than nothing when out and about and even in the circuit, but could still cause confusion in the circuit. My loyalty to OzRunways is unabated.