I'm in Thornleigh, NSW area and recently I got constant very slow upload speed on HFC NBN 100/40 plan. now only around 2-3Mbps. I tested many times in different time. Even in off peak time, I still got the same result.
As a professional network engineer for more than 10 years, I have checked my home network, router and other devices and can be sure that it is not an issue in my home devices. It is definitely a throttle on Optus side.
Tried to contact Optus, but the response is way too slow and after waited hours, the message I got just suggest me to reboot he router/modem. That really sounds like a suggestion from a non-technical person.