Maybe thats just me though. My Thai phone provider gives me global roaming with unlimited data capped at ฿300 a day (about $10 a day). The last time my wife and I came back to Australia together, we had pre-paid Telstra sims. The time after when I came alone for work, I didn't bother, as the roaming cost worked out better than Telstra's shitty pricing, and I still got to use their network.
As for an actual solution, I doubt there is one an ideal solution in this case. Honestly, I would want to need access to a lot of other people's Wifi before I signed up for this type of deal - how sure are you this doesn't give the other Telstra members carte-blanche access to your local network, and how sure are you a judge will accept "but your honour, that could have been anyone downloading those pictures, I have Telstra Air".
