For NBN services that are installed with an NTD, there are specific speed limitations associated. This is primarily for customers intending to provision multiple services (on different UNI-D ports) on a single NTD.
The limitation on the NTD, as it is classified as Traffic Class 4: Up to 1000/400 Mbps. It is not a limitation on the network. Basically, the NTD will only support up to 1000 Mbps in total.
Example:
2 NBN services, each with 500Mbps, or
1 NBN service with 1000Mbps.