In the past one needed a modem to connect to the internet, and a separate router to create a Wi-Fi network so your devices could connect wirelessly. These have since been combined into one device and what we now frequently apply to as a modem is for the most part a modem router, though it is still possible to get standalone routers.
If your nbn connection type is Fibre to the Node (FTTN) or Fibre to the Building (FTTB), you need a modem router, but if it is Fixed Wireless, Fibre to the Curb (FTTC), Fibre to the Premises (FTTP), or Hybrid Fibre Coaxial (HFC), you only need a router as, the nbn Connection Box that will be installed in your home acts as a modem. You can still use a modem router with Fixed Wireless, FTTC, FTTP, or HFC, but the device must have a WAN or Internet port.