On the other hand: a device should not join too often (possibly only once in its lifetime), and the procedure to join is well documented. For 1.0.2:
If using the over-the-air activation procedure, the end-device should broadcast the JoinReq message alternatively on a random 125 kHz channel amongst the 64 channels defined using DR0 and a random 500 kHz channel amongst the 8 channels defined using DR6. The end device should change channel for every transmission.
Unfortunately, the AU915 specifications are a bad copy-paste from the US915 documentation (sometimes even still referring to the US tables), but for US915 this explanation is a bit more extended:
For rapid network acquisition in mixed channel plan environments, it is further recommended that the device follow a channel selection sequence (still random) which efficiently probes the groups of nine (8 + 1) channels which are typically implemented by smaller gateways (channel groups 0-7+64, 8-15+65, etc.).
So yes, joining might take some time, but due to using DR0 (best range) chances are good that when a randomly selected channel is indeed supported by the network, the Join Request will be received.
Next, while 1.0.2 does not support a CFList in the OTAA Join Accept yet, the initial ADR will tell the device which channels to use. (In 1.1 TTN will include the network settings in the Join Accept, like it already does for EU868.)