When you join the NBN with Optus they will provide a new modem if needed.
Depending on what technology NBN you have (HFC cable, FTTN, FTTP, FTTC, etc.) you might need a different set up. In many cases NBN will provide a second dedicated modem to go between the wall plug and your existing modem (plug and play with one cable in and one cable out). But as mentioned if you need to replace the existing one then Optus will do this.