Telstra's website has an area you can go to to retrieve your PUK code at any time. A PUK code is something you have to enter after you've tried entering your SIM card's PIN code more than 3 times. After you enter your PUK code, it will give you the chance to set a brand new SIM PIN code.
Residential and Business customers have different links (both of them are on that one web page). It's a quick and handy way to get your PUK code without having to call anyone or visit a store.