ADSL speeds are determined by the distance from your exchange.
Line speed can be affected by line faults; TPG can check this.
Can you run the speed test on TPG's support page? Use a computer connected to router by cable.
Run it on wifi device and see the difference, especially if the wifi device is some distance from router.
Check on TPG's coverage map for ADSL2+ to check the capability of your local exchange and get a rough idea of your distance.