Anna, If you are referring to the Modem supplied to each of TPG's NBN Plans, you will be able to connect to this wirelessly. The modem will come out pre-configured so you will plug the modem into either the Wall Plate, NTD or Existing Phone Socket (depending on how NBN is delvered to your home) and then connect wirelessly to your Mac.
Alternatively, if you do wish to connect with a cable, you should be able to purchase a USB cable adaptor for Apple products from most electrical outlets like Harvey Norman, JB Hi-Fi or online from Ebay.
Hope this helps.
Welcome to TPG Community!
@DB is correct. The modem/routers that we are providing our customers are pre-configured and you just need to connect it to its designated port.
If you wish to change the settings on your modem (such as WiFi name and password, etc.), you can do it wirelessly. You just need to connect to the current WiFi settings of your modem/router, then access the modem's interface. Try this link.