Thanks all for your advice. Just as an update for anyone else who's in this situation again.
For the bank transfers issue, I'm going to do a few things:
First up, I've ordered a free sim card from 3 in the UK, sent to my inlaws address in UK. The sim card needs to be put in a phone in the UK in order to be activated first, before it can be used overseas. My father in law has done this now and is in the process of posting the sim to me here in Sydney. My intention is to try change contact numbers on AU bank and other accounts to this UK number in the week before I leave - however as CupFinal73 notes, my bank might tell me to go jump when I try to change to a UK mobile number so I need a back up option
As a result - and thanks Marisa for the suggestion, I am going to request a netcode token from Commbank. Their website states that netcode tokens are only provided in 'exceptional circumstances' (in other words they want to discourage usage but will give you one if you really need it) - so I suspect I will need to be a little creative with my reasoning to ensure they can't say no.
As a double back-up, I'm going to set up BPAY with my FX provider as a payee (I will do a couple of low value transfers to test this out before I leave AU). Thanks AliQ and others for that one.
Across all of these, I feel like I have a few bases covered, and less stressed now about having issues getting the remainder of our money home in the months after we have left.
My next target is now to figure out a way to access myGOV online after I've left, as I just remembered that also has a SMS verification process for sign on! There's a 'netcode' style app apparently but the reviews are brutal so going to see if I can find another workaround before going down that hellhole.
Cheers all! Will update if I find anything useful.
Gerry