The Australian players almost always use the true continental grip. The hammer grip is usually preferred by those who for whatever reason have weaker wrists and require more support. It is actually easier to become familiar with and use the hammer grip. The tighter your grip with these three fingers, the more stability in your wrist when you hit the volley. Rather, you want to tighten them at the moment your racquet makes contact with the ball during the volley.
Stance is next.
The key is to try and be balanced during the volley. Stay a bit low with knees bent. Keep your back straight. If you do these two simple things, I promise you that your volleys will soar. Frequently, you'll hear commentators and instructors refer to the split step when approaching the net to volley.