Probably having a word with support about any possible congestion issues in your area would be the first step. You can do that via the live chat link below if you wish.
What NBN technology? Fibre?
As @Yeldarb says a chat to support might flag something (Maybe a NBNCo issue) and you could find the issue addressed at least somewhat.
If not then its most like Optus not providing sufficient bandwidth for all users in your area at peak times. You can't really do much about that unfortunately. Optus should let you out of your contract without penalty if they aren't supplying what they are charging for. After that its anyones guess who might be a better provider - see what the rest of the internet suggests.
Note if you have an Optus Fetch you will need to return that.
Same thing happening to me in the Glen Waverley/Wheelers Hill area with HFC NBN. I pay for 100/40 service and between 6pm and 11pm I get 4/30 ...that's 4Mbps!!! I can't stream at that speed and forget about using Fetch. It's downright disgusting! It's like the dark old days of cable when the service was oversold and there was terrible congestion every night!!!
IMO the "I pay" bit is the issue. You're no longer paying what is required for high speed download. Optus is fudging the issue (and not offering an alternative plan). Do some googling. If you're willing to pay $20-$40 more then you can have what you're paying for.