That's not good, GatorBoy.
Info about this can be found on our help and support page: Lost, Stolen or Found your Mobile Device?
I just learned this can be done online by logging into MyAccount:
We recommend blocking your device and your SIM card.
Blocking your SIM will prevent any unauthorised usage and additional charges on your account.
Blocking your device ensures your mobile phone can't be used on any Australian mobile network with another SIM card.
Note: It may take up to 24 hours to block your mobile phone and SIM card. If you have AutoRecharge active, you'll need to cancel it manually. Check out how to do that here.
Here's how to block your device and/or SIM card: