I have checked your NBN-FTTP 12/1 service on our systems.
FTTP is normally pretty reliable and all tests on our end are passing.
I think you are using an older NF-12 modem. It is pretty old now and would strongly suggest to use another modem which could fix your issue.
Also, 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.