A single like the NX650 (called "the dominator", or "dommie" in most places) is not the first choice for either two up, or for long highway miles like those you face in Australia. It is a good choice though if you are planning on taking dirt tracks. A compromise, like most of the 650cc singles overlanders favour. A fast comfortable off-road machine doesn't really exist. It can't.
Australia is a difficult one to call, because the highway distances are so vast, but the dirt track opportunities are excellent. You'll have to think about your priorities. A big old fared thing would do the job well, but exclude some of the interesting dirt-road spots. Similarly, a 250 D/S would be a hoot on dirt, but tiresome after days on tarmac. That's why most of us plump for something in the middle, like, humm, an NX650.
The dommie has an excellent reputation, and you wont go far wrong with one. But forget the speedy touring. Most loaded-up singles of this size like to sit at about 100km/h and not much more. Anyway, what's the rush... relax, smell the marsupial roadkill.
Around the world 2000-2004, on a 1993 Honda Transalp