Hail is a type of precipitation or water in the atmosphere. Hailstorms are formed by layers of water attaching and freezing in a large cloud. A frozen droplet begins to fall from a cloud during a storm, but is pushed back up into the cloud by a strong updraft of wind. When the hailstone is lifted, it hits liquid water droplets. Those droplets then freeze to the hailstone, adding another layer to it. The hailstone eventually falls to Earth when it becomes too heavy to remain in the cloud, or when the updraft stops or slows down.
For more detailed information about possible storms in your area, check out the Australian Bureau of Meteorology here