If you can find a place between the two terminuses of this link which has good visibility of both, you could perhaps (if you could get permission) mount a reflector there to complete the circuit so to speak.
Similar to how this is done on microwave towers. It could be something as simple as an old highway sign or similar. (a strong, flat piece of metal sufficiently large.) just mount it so securely its not going to move significantly in even the highest winds. Adjusting it properly might be difficult - thats where having he best possible elevation data comes in. (see below)
One thing that must be truly awesome is that Australia being south of Japan, also has coverage from the Japanese QZSS system and Beidou, the Chinese GPS system. Its my understanding that far more sats are visible in Asia for this reason. Just use a newer GPS that supports them.
I wrote a long post about RTKlib and hardware that supports it and setting up an RTK system but it may not be necessary IF you use a system that supports the newer sats available in your area. Thats important because the *vertical* accuracy of older GPSs often leaves much to be desired, that is to say its often way off. (typically the offset stays the same in a given area) For some of my old GPS's the vertical direction is consistently overestimated. My real elevation as measured properly is substantially less than they tell me. This has to do with a bunch of factors, which you'll learn about if you explore RTKlib. (which is also a lot of fun learning)
Anyway, with that caveat getting an accurate position when you have a clear sky view is fairly easy and with the multi-constellation units is fairly rapid, even without RTK. especially if the unit has been running for awhile with its antenna having a good view of the sky (on top of the car that gets you there?) Make sure your fix quiets down after you turn the unit on have it still and running for awhile with a good sky view before taking your measurements.
So anyway, one of my GPS's that seems extremely accurate and supports GPS, QZSS and Beidou - my navspark mini, cost me less than $10 and gets Beidou sats (only one or two are ever in view here in he US) But those sats add to the quality of the fix significantly. Enough for the indoor movements of the GPS's antenna to seem accurate. (for example, moving the antenna from next to the window to computer desk, a distance of around two meters, was recorded with what appeared to be fidelity) This is without DGPS of any kind, inside of a two story wooden house with the usual levels of wiring and stuff in it. That's very impressive to me. So, you likely would get even more accuracy, being in Australia because of the way these newer GNSS satellites orbits are designed to keep them over one part of the world, as the earth turns beneath it. The QZSS system was designed to augment GPS for very densely populated Japan, and he fact hat Australia is directly south of Japan means it spends as much time over Australia as it does Japan. So you get that extra accuracy for free. Similarly with Beidou and China.
Record a log and record the time so later you can correlate the two and when you get home figure out what the exact heading and elevation the links between them would need to be.
Do this by making a coverage map in one of the aforementioned programs using the exact same scale (and if you ca perhaps also the same coordinates to define the image borders) and then overlay the two in an imaging program and "diff" them, start in the "difference" layer mode perhaps and the areas that are covered by both signals at the highest level should be visible.
Those places likely would be good spots for a reflector!