I am having the same issue.
Recently had NBN connected. Telstra customer service tell me it is going but my speed is the same as it was with adsl2.
Did the speed test and get 20mbps download. My problem is, I have an NBN box on my front lawn and a hole full of Telstra cables so it's literally 7-8 metres from the hub to my computer. But yet the speed is still nothing more than I had with adsl.