***These instructions were provided by a member of the community and are not officially supported by Optus***
Thanks to craiglaw76 and Mycal, here are some instructions that helped them to enable the use of the USB port on their modem.
1 - In your web browsers address bar, type 192.168.0.1 (or what ever your modems IP address is)
2 - Click advanced on the left
3 - In the advanced page, click on "Multimedia" and then "Storage Device"
4 - Take note of the volume name of the drive connected. In my case this was "usb1_1"
5 - On the left, click "User Accounts"
6 - In the main section of the window, click add.
7 - CREATE a username and password to use to access the USB drive and enter the USB volume name previously noted. Apply and Save once done.
8 - You wil be taken back to the user accounts overview automatically. I enabled "Samba Server" but am not sure if this needs to be done. I have not tested with it disabled.
9 - For Vista/7 and maybe 8/8.1, Click the Windows Start button and immediately type in "\\192.168.0.1" (without quotations) This will load Windows Explorer and prompt you for Username and Password. Alternatively you can use: Start>Run or Windows Key+R or simply open "My Computer" and type in the address bar at the top.
10 - Enter the username and password which you previously created (it helps to tick "remember my credentials" for future use)
You now have access to the USB Drive connected to your modem/router.
You will notice the shared folder will be named the same as your username.
You could even right click on this folder, and choose "Map Network Drive" and assign it a drive letter so that it appears in the regular "My Computer" window.