Hi there
I am sorry to hear that you are advising there is an issue with the speed of your NBN FTTN SL service.
I have had a look into you account and conducted various testing and everything looks to be within specification and testing good..
Your current NBN FTTN SL service plan speed is 12/1Mbps
That said, the modem itself will be getting a sync rate of 12/1Mbps in the GUI (graphical user interface) and then will always encounter some sort of loss to transfer it to your devices. How much loss depends on what your connection type exists between your modem and your device, the most common is WIFI...
With a WIFI connection you will commonly get a normal loss of up to 2Mbps for a 12/1Mbps service. You can try reduce the loss possibly with a wired (ethernet conection) connection however some loss will always be present ..
Given your current plan and your speedtest results i would advise you that it does appear to be testing well within specification. If you do require faster speeds i would kindly recommend increasing your plan to the next step up and see how you go.
Also if you like to double check your connections both wired and WIFI and maximise your speeds please vist the useful links i will provide for you below.
To improve your WiFi connection, follow the steps in our TPG Community article at www.tpg.com.au/wifihelp
For helpful tips on how to improve your internet speed, follow our guide at www.tpg.com.au/slowspeed
I do hope that helps 
Kind regards