The technology plays a part but they're in 4 main groups.
1. Fixed Wireless is its own category and has data and speed limits (and often congestion)
2. Then comes FTTN. This is basically like old ADSL with the closer you are to the node deciding how fast you can go. Roughly 50% of houses can get 50-100Mbps. The other half between 25 and 50 Mbps. Its a lottery.
3. Next comes HFC Cable and FTTC. HFC just uses the old HFC network and can do 100Mbps+ easily enough. FTTC is new (again sort of like ADSL but the 'node' is right outside your door.) FTTC can also easily do 100Mbps+
4. And of course FTTP. Which is what we all should have installed as it can do 25-10,000Mbps you just have to turn the appropriate dial. 20% of households are FTTP.
Unfortunately over 40% of households are FTTN and they lost the lottery (although a fair few of them can still get 100Mbps because the node isn't too far away)
But about 70% of the population should be able to hit 100Mbps if they want too. As an aside the NBN is now tweaking Phase 2 of the NBN and all HFC users will be able to go 250Mbps within the next 12 months (70% can now)
FTTP all have access to 1000Mbps but the pricing is bad ($150+) and also the upload speeds are hobbled.
PS The original poster was FTTN and had good speeds already. They just dropped for a yet unknown reason.
So have you progressed this @kyriacandro? Are Optus saying the top speed is 16Mbps and they won't do anything else?
First thing is to just confirm Optus haven't cut you down from NBN50 to NBN25 themselves. They might have done this automatically when they go the NBN line test. So confirm the NBN50 pack is still applied.
If that's not the problem then they need to organise an NBN contractor to look at your line.
What postcode / nearby street are you in?
I don't know how much more clear I can be with them
Then you need to either escalate it to a manager or tell them that you want to make a formal complaint. And if that fail, tell them you will complain to the TIO (Telecommunications Industry Ombudsman), that will certainly get their attention.