The phones are only through Telstra. In theory, they should be locked, but in reality I don't believe that stock was network locked.
My theory? Apple sold Telstra a tonne of (discontinued) 32GB 5Cs, and as per Telstra policy, they have to be activated at POS. JB are a Telstra dealer, so it's the same rules as it's not Apple stock - it's Telstra stock.
Take the SIM out and snap it, problem solved.