There is no minimum thickness in QLD. NSW has the highest requirement, which is 6mm, so most manufacturers make them to that standard as they can advertise them as Cat H compliant no matter where they are sold. 2mm is fine, the gun shop just wants to sell you something they have in stock or can order in. If you really wanted to you could go down to Bunnings and grab
https://www.bunnings.com.au/sandleford-gem-anti-theft-digital-safe_p4220419. I wouldn't, but it is perfectly legal in QLD as long as you bolt it down.
QLD regs just state:
In a solid steel container, bolted to the frame or floor of a permanent building.
Container must be kept locked with a sturdy combination lock, keyed lock or keyed padlock.