You can add a new mailbox/user by logging into your mail admin control panel at http://mail.yourdomain.co.nz/mailadmin. Click on "Manage Accounts" from the menu on the left of the screen and then click "Add User" to create a new mailbox. Fill in the details as necessary:

Full Name: This is a description of the account (e.g. Sales Department, Bob Smith etc.)
Address: This is the email address that you would like to have (e.g. sales, bob etc.)
Password: A password of your choice (preferably 8 letters minimum, containing at least one uppercase letter and one number)
Verify Password: verification of the password you entered.
Then click on “Advanced Options” and set the “Mailbox quota:” to the required amount.

Then click the Submit button to create your new account.

Please also see the Adding an Email Account Guide

[Note: Replace any instances of yourdomain.co.nz with your actual domain name]