Did you check for outages in your area?
Click outages Fixed Outages for all NBN connections and enter you postcode.
If you are connected to the NBN you can also check here
https://www.nbnco.com.au/support/network-status you have to enter your full address.
If it is an NBN issue depending on where the fault lies, if it is on the NBN side of the network Optus and all other RSPs have to wait for the NBN to repair the fault.
If there are no reported outages have a look here at some troubleshooting tips