If it's just a stop to rest and recuperate: Gundagai is about half way. There's a hotel, a motel and an RSL for cheap meals. You could push on to Yass which is another hour or two closer to Sydney.
If you want to make something of a stopover, then get to Canberra. Lots of art galleries and cafes and interesting things to do for a day, and then only 3 and a half hours more to Sydney.
It's possible to do the whole 9 or 10 hour trip in one chunk, but watch out for driver fatigue - make sure you swap drivers every couple of hours. If you decide to do it overnight you'll be sharing the road with lots and lots of very large trucks and it gets a bit Mad Max-like! Safer and less tiring during the day.
Phil at the Safety Hub about 8 years ago