The equipotential surfaces corresponding to a field that uniformly increases in magnitude but remains in a constant z-direction are

As the field is having constant direction in the positive Z-direction, the planes which has normal along Z-axis will be having same potential.

But as the electric field is increasing in magnitude, the separation between the equipotential planes will be unequal for the same potential difference between them. $\left(\because -\frac{\mathrm{dV}}{\mathrm{dx}}=\mathrm{E}\right)$