Same happened to me in transit at SYD when flying NZ to BKK. (no duty free on Qantas flight WLG-SYD) Moved 20m up the escaltor and I could buy as much duty free as I liked ....
Now, flying BKK-WLG on Qantas flight, I transitted SYD, and passing through security is a sign saying duty-free OK (duty free available on Qantas fliht BKK-SYD).
This has nothing to do with security - Qantas and SYD airport ring-fencing duty-free sales.