Thank you for being a part of the TPG Community and for your question today.
Your current wiring for VOIP services from your OPTUS Cable connection can be used on your new NBN connection.
Your Optus Cable service will become obsolete and will not be used.
You will require a copper POTS (Plain Old Telephone Service) connected from the street to inside you home, ideally where your Optus service is now so you can simply transition to NBN.
If you hold a ACMA licence you should be able to do this unless it requires an aerial leadin which would require the Aerial Endorsment on the ACMA lience.
I hope that helps.