First way: look at the formula for the electrical field gradient tensor. We see that Vxx = Vyy, so we have the asymmetry parameter eta = Vxx-Vyy/Vzz = 0, so we have axial symmetry.
Second way: look at the picture of the system, it will not be physically different if we rotate the system around the z-axis, so we can say it has axial symmetry around the z axis.