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For newly installed NBN service, we did an article that you may find helpful.
In the event that you successfully set up the NBN service and still not working you may check if there is an ongoing outage affecting the service. See this link for reference How to Check for NBN Network Outage.
If there's no outage, you may perform basic troubleshooting step available in this article Troubleshooting no internet connection.
In this particular connection issue affecting the service. The modem/router has been properly setup and configured however our initial test result shows a multiple dropouts from the modem/router to our network.
This has been raised to our Technical Team for further assistance, one of our Technicians will contact you tomorrow between 10-12NN AEDST.