Had a chat with the Shop Manager. The ID under $5000 is really only if the transaction/behaviour is suspicious. Something that would be suspicious is (actual case) guy came in with three other family members and they all purchased under 5k and then turned up the next day to do the same thing.
From a staff members point of view it is very difficult to judge when a customer is deliberately trying to avoid providing ID and who is just genuinely trading at different times.
Without knowing the circumstances of your purchases, it may have just been an overly zealous staff member - just because you are regular sub $5k customer doesn't mean we need your ID.
We are having a training session for staff on what is "suspicious", so hopefully that will result in consistent application of our policy. There is a sign in the shop stating how we handle repetitive transactions under $5k.
If you think the ID request is unjustified, just ask to speak to the supervisor/manager.