Have you plugged the Wan port of your old router into a Lan port on the new router? If everything is working and is satisfactory with this configuration then the main disadvantage I can think of is double Nat. Some people are apparently using this setup with Telstra as a work-around to use their own router for internet. However, without the Telstra provided router NBN home phone won't work. Telstra is refusing to give out VOIP passwords to set it up on other devices. Apparently this setup works surprisingly well but can cause problems with some services, including at least some VPN's. If you're using wireless you now have extra wireless networks, which can theoretically cause problems but probably wont.
Unless you have some good reason for wanting to keep your old router I'd personally recommend you retire it and just move your network to the new one. However, if you can't be bothered and dual Nat is not causing you problems there is no compelling reason why you can't leave your setup as is. Having said that, I've plugged my old router into the modem (cable in my case, not NBN) because I use features which are crippled or not available on the one provided by Telstra.
Bookmarks