A neutral conductor has equal amounts of positive and negative charges in every small volume or surface element.

When the conductor is charged the excess charge can reside only on the surface in the static situation.

Let us consider a Gaussian surface inside the conductor and close to the surface.

At all points inside the conductor $\overrightarrow{\mathrm{E}}=0$ hence from $\phi_{\mathrm{E}}=\int \overrightarrow{\mathrm{E}} \cdot d \overrightarrow{\mathrm{S}}, \phi_{\mathrm{E}}=0$

According to Gauss's law,

$\phi_{\mathrm{E}}=\frac{q}{\epsilon_{0}}, \phi_{\mathrm{E}}=0$

$\therefore q=0$

Hence, there is no net charge at any point inside the conductor and any excess charge must reside at the surface.