There are some things that the 7G nano doesn't have that could be dealbreakers for 6G owners. For one, the 7G doesn't have a clip. Another is that the 7G can't be used as a watch.
The 7G does bring some cool stuff to the table, though. Bluetooth, full Nike+ support (works with sensor and HR strap; perhaps not the remote though) without dongle, video capabilities, etc.
Only you can decide whether it meets your needs.
As for the 6G, how is it broken? Is it just a dead battery. Did the buttons stop working? Not sure about the warranty laws/rules in Australia, but in the US, Apple has a program to replace* dead batteries for $59. (* - replacing the battery means giving you a new/refurb ipod).
Perhaps if you have a dead battery, Apple Australia has a similar program. If it's the buttons, you may want to see if Apple will give you a replacement for a nominal fee. Or you can fix it yourself as the button problem is sort of common and someone with a moderate skill set can fix, provided your OK with a heat gun to loosen the glue.