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For customers experiencing any issues with their NBN service, we recommend the best thing to do first is check if there's an outage on the NBN network.
We have created this article that can guide you on How to Check for NBN Network Outage
If there are no known outages in your area, customers can proceed with the troubleshooting steps indicated on this link : https://www.tpg.com.au/support/nbn-fttn/problems_speed_nbn_fttn
We've created an article that will guide customers to troubleshoot a Slow speed issue.
In your case, you are under the NBN 12/1 speed tier and the speed that you are getting seems within specification. We understand that you've upgraded your plan to NBN 50/20 speed tier and is still in progress. Further updates will be given once available.