ADSL and NBN both use DSL technology to deliver broadband internet access over existing copper telephone lines. There are two major factors that govern the achievable sync speed of DSL technology
Distance from the DSLAM
The actual cable length between your modem and its counterpart, the DSLAM is the main factor for achievable speeds. For ADSL, the DSLAM is usually installed at your local Telstra telephone exchange building (those sometimes creepy old 1950’s buildings that look abandoned).
For NBN FTTN, the DSLAM is inside the node cabinet.
VDSL uses radio frequency to deliver the signal over copper. This means that the further you are away from the DSLAM, the more signal will be lost in the higher frequencies which will reduce your useable bandwidth.
There is not much you can do about this apart from moving to a premise closer to a DSLAM.
In-Premises Wiring Configuration
Homes and premises with more than one telephone outlet may experience a further loss of bandwidth due to the fact that for optimal data transmission over a copper line the circuit should be point to point. In other words, there should only be two terminating points to the circuit, the DSLAM and the modem. When multiple terminating points (sockets) are present, the transmitted data travels down each path, and will bounce back from the unused termination points and collide with the main stream of data.
If you find yourself in this situation you should consider engaging an ACMA registered cabler for a broadband rewire. This entails a new cable being installed from where the service enters your building to where the modem is located.
If good quality cable such as category 6 data cable is used, this will give your premises wiring greater cross talk immunity and protection from radio frequency interference which can be generated from such items as down light transformers, exhaust fan motors and the like.
The cabling needs to be installed away from 240v wiring as much as possible and should be properly secured to beams or framework inside the premises. The cable should be join free inside the roof space to maintain crosstalk immunity.
Finally, the benefit of ensuring your premises cabling is 100% compliant with todays standards is that in the event of service interruption your service provider cannot blame the problem on wiring at your end and is then obligated to rectify the issue in the network. Ensure your cabling provider gives you a TCA1 certificate of compliance (they are legally obliged to) which you can present to your ISP if you encounter service issues.