I have checked your NBN-FTTC 12/1 service on our systems and all tests are passing.
A 12/1 plan with heavy household usage can appear slow at times because the 1meg upload speed becomes a bottleneck.
For heavy usages, a 50/20 plan will provide much more reliable speeds to heavy usage situations.
Here is a doc to help TS speed issues.
I have also passed your details on to an accounts manager and they should call you back ASAP.