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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, then you may proceed to the troubleshooting in this article: Slow speed issue
If you are using a WiFi connection, then you may check these articles that will help you improve your home network. Here are the links: