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There are many factors that can affect the speed of an NBN service. Depending on the access technology, the actual throughput speeds may be slower and could vary due to many factors including the type or source of the content being downloaded, the number of users and devices that simultaneously use the service, the way the service is being used from wired or wireless connection, the number of users that is connected using the network infrastructure for different service.
We've created an article that will guide you on how to troubleshoot a slow speed issue. Check it here.
If you are using a WiFi connection, then signal interference may have caused the issue. We have some articles that will help you improve your home network.
Check these articles:
Let us know if you need further assistance.