Welcome to TPG Community!
I checked your account and learned that your modem is receiving a passing sync speed with a Downstream of 11 Mbps and Upstream of 1 Mbps (You'll see it on your modem interface). The download speed of 5 Mbps - 10 Mbps that you are getting via speedtest is passing based on your sync speed.
You are currently 2.1 Km away from the local exchange which is in Burwood. You can check your address on TPG Maps for more information.
Unfortunately, that's the best speed that you can get for ADSL2+ connection due to your cable distance from the local exchange.
Other providers of ADSL2+ are using the same infrastructure that we are using to provide you the service. There's a possibility that you'll get the same speed just like what you are getting for ADSL2+ connection.