I have checked your NBN-FTTC service on our systems, and is passing in the NBN portal.
The 12/1 speed plan can feel slow at times because the 1meg UPLOAD speed can bottleneck quickly with heavy usages.
The next speed plan 50/20 is much better suited for high usages situations, and really opens up the potential of the NBN network.
Keep an eye on the BLUE LIGHTS of your NCD. If they go off or flash, let us know.
If they stay on and are stable, it may be a bandwidth issue.