Join the ranks of p'd-off NBN customers with Optus! I feel your pain. I'm on a 100/40 plan and right now in Glen Waverley Victoria my speed is down to 3Mbps!
In my case a senior technician has confirmed that the problem is CONGESTION caused by Optus not having enough bandwidth to service all their customers in my area...in other words Optus is still selling their speed packs, etc... in my area knowing, full-well, that they have not got sufficient bandwidth to cope the demand on the network in the evenings. What's the bet that's happening to you as well! Mate...start complaining. Lodge a complaint with the TIO here: https://www.tio.com.au/making-a-complaint
Optus needs to be made to understand that they cannot continue on selling these packs and thinking people will tolerate the congestion and lump it. They MUST fix the bandwidth issues.