When installing sinks and cooktops, you'll generally have to cut through not only the benchtop, but also the support rail of the cabinet directly below.
One way of avoiding this is to rotate the support rail to a vertical position prior to installing the cabinet.
How to do it yourself
Simply mark a line 8mm back from the front edge of each side panel. Using a countersinking 4mm bit, pre-drill 2 holes; 24mm & 56mm down from the top of the cabinet. These holes should line up with the pre-drilled holes in the support rail which you can then screw into place using the screws provided.
DIY level - Beginner