Sorry, just re-read your original post. You need the country bit for the wireless channels.
See if you can't turn off all the extras on the optus modem, so your router gives out the ip addresses and does the routing - if that doesn't work too well, try it the other way round. We brought over a Belkin wireless router ourselves but it has a dedicated access point mode, so our modem/router does all the hard work. Does the optus device have a model number or maker? Let me have the same for the Netgear.