Surprised this has not been posted from a quick search.
This started a few years ago and its still active, although its a little hit and miss, and it can take more than a week to obtain the free Medium pizza. But free pizza tastes good.
You need to complete a survey to get the free pizza, after ordering pizza from a Pizza Hut Restaurant within two days of the order.
If you order online, the survey link will be in the email receipt. If you order in store, the paper receipt will have a code for the survey which you can enter in link https://s.pizzahutsurvey.com/aus
After filling out survey, wait for email which has a code for a Medium Cheese Pizza which can be used for an online order.