We'll be taking anything of value out of the packs and in hand luggage, but any suggestions on how to secure against any theft from the packs that will be in the hold would be appreciated.
Nothing beyond what you have done! I have my key documents and difficult to replace items like my phone, medications and camera in carry on, and everything else gets checked. I doubt there are thieves operating in aircraft holds, but I expect you are really thinking of when the bag is being loaded, trans-shipped and unloaded
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I have taken a variety of approaches to ensuring that the bag has no loose straps that might get caught in airport luggage handling machinery, including some of the measures already described. I doubt they are robust enough to completely eliminate theft, but they might offer some additional security. For a start, they protect the bag from visual inspection that might identify it as a target. And then there is then another physical layer that would impede anyone trying to open the bag itself, although where a baggage handler is going to find the time to do that surreptitiously is a bit of a puzzle to me.
I think there is a greater risk that a bag is delayed, or gets lost for some reason. Not impossible, but in now just over 40 years of international travel, I have been fortunate enough to have a bag delayed just the once, and it arrived the following day. Domestic travel is another matter, but I think that most major international carriers have baggage handling systems that are now well up to the task of keeping passengers and their luggage together, whether domestic or international, and have been so for a long time. Perhaps there are carriers and airports that aren't as good, but I don't think you will face that problem flying from Australia.
Having been delayed once, the other close call was not on the camino but when my wife and I were returning from overseas. My wife had brought a pretty distinctive bag so it would be easy to see. We saw one like it drop onto the luggage carousel and as we waited for it to get to us, someone stepped forward, took it and was headed to the customs/quarantine checkpoint. It took me few moments before thinking that I ought to check that the woman who had collected it had checked that it was actually hers. I'm not up to much dashing after a long overnight flight, but I did then, and fortunately caught up with her and, much to her embarrassment, recovered my wife's bag. Her thinking had been much the same as ours that there wouldn't be anyone else with the same colourful butterfly print pattern, and hadn't bothered to check the baggage label.
So yes, there are still risks, and it would be wise to be prepared for the loss of your bag even if it is a remote prospect. First, have insurance to cover this. Talk to the baggage service staff if your bag doesn't arrive and report it. If you are lucky, they may already know where it is and when it will arrive. Contact your insurer if you need to spend money getting new gear, etc. Reshape you plans. None of this is painless, I know, but none of it is impossible either.
Finally, I also wonder why you don't think you will get a 36 li pack onboard as carry-on luggage.