Hi Sunny_C,
If you can take a look here - http://www.nbnco.com.au/connect-home-or-business/information-for-home-or-business/will-it-work-over-the-nbn/device-compatibility.html Specifically the section for 'Fax and Teletypewriter devices'. Depending on the type of fax you use, it may not be an issue running it from the green voice port on the modem. Please call the manufacturer of your device to check for compatibility.
If the device is not NBN compatible, please call our sales team on 1800 834 273 say 'Sales' and we can arrange for IP Pots to be connected.
Nick