Have you changed your plan? Also it depends on what type of nbn you are connected to, like fttn/fttp/hfc/fttb etc.
Check with your neighbours too 🙂
Also, please give me a kudos if this helps please...
Sorry I forgot to mention these are approximate speeds, and 16mbps is the max off peak speed for speed pack 1
Where are you getting that max speed pack 1 info? Speed pack 1 (NBN12) is max 12Mbps so should be be getting anywhere near 16Mbps. The Optus typical evening speed is 11.1 as shown here.