If you drop a paper which is a pre-requisite for Semester B, you will no longer meet the entry requirements for that Semester B paper, and will need to defer taking it until after you have completed the pre-requisite.
Support services are available which may help you continue with the pre-requisite paper, instead of dropping it. Talk to your Division or Faculty or the Student Centre about this, and your other study options. You can contact the Student Centre by logging an enquiry, or by calling 0800 WAIKATO (0800 924 528) or +64 7 838 4610 from overseas.