Welcome to the community!
We are able to locate your account using your community details. Due to Copper Line's Limitation the maximum sync speed you can get from your NBN-FTTN service is 30mbps.
We'd like to confirm to confirm on what device(s) you are able to get 18-23mbps of connection speed. If this is on Wireless device(s) we did an article that will help you improve your wireless connection available here.
Have you tried to run a speed test using a desktop/laptop connected via LAN/Ethernet cable? If so, please post the speedtest result on this thread for additional reference.
Note: Devices connected by Wi-Fi may experience slower speeds than those connected by Ethernet cable.
Since you are on a higher speed NBN plan, you have the option to downgrade to a lower speed plan without any penalties. Let us know if you wish to make changes in your account and we'll have one of our Account Specialists contact you.