When you build a water storage tank, it is important to build the base well. It is the first section that touches the ground. The size of the base is directly proportional to the actual size of the tank. If you fill the tank, then it becomes quite delicate. Due to the pressure exerted by the water, it can easily break.
Several factors play an important role in making the base:
Size
Stability
Type of the base
Nature of the ground
The steps of building Canberra water tanks are explained in this blog.