You can make these changes yourself at https://secure.freeparking.co.nz. Click on 'Login' in the options at the top of the page and then login using your username and password.
Click 'Services' from the main menu.
If you have more than one domain under this account you will then need to click the 'Manage' button beside that domain. Then click the 'Service Management' tab.
Look for the heading 'Domain Name Servers' and click on 'Modify' then make the appropriate changes, followed by clicking the 'Continue' button provided.
For a .nz name the change will then be updated at the next NZ Registry Zone rebuild which happens hourly. Full propagation can take up to 48 hours.