If you exceeded your monthly download quota your connection speed will be throttled. You can view your recent download history and total downloads for the month, by logging into your TPG account online at https://cyberstore.tpg.com.au/your_account/ . We recommend that you monitor your usage regularly.
Please let me know should you require further assistance.
You haven't specified what type of service you have.
If you're on a fixed line service (ADSL, ADSL2+, NBN) and you happen not to be on an unlimited data plan, then your speed will be slowed down (shaped) for the period in which the monthly usage quota has been exceeded. You don't get charged for additional data.
If you're on a mobile plan and you exceed your plan's Included Mobile Data value, then additional excess data usage charges will be payable in accordance with the terms of the plan that you have. For more details on these charges, go to https://www.tpg.com.au/mobile > Important Things You Need to Know > About your Mobile Plan.