A UDAI code (pronounced you-dye) is a unique 8-character 'password' for .nz domain names. This code allows you to transfer your domain from one domain provider to another. UDAI codes only include numbers and letters and are case-sensitive.
If you have been given a UDAI code. for your domain, you can check it's valid by using this tool: https://www.dnc.org.nz/udai.
Some domain providers have different names for the UDAI code, such as a Registry Key, or a Transfer code. To get your UDAI code you can simply ask your domain provider, however, once you receive it, ensure that it's actually a UDAI code by using the tool above (https://www.dnc.org.nz/udai).
If your domain is with Rocketspark, you can get your UDAI code by following this help guide: How do I transfer my domain away from Rocketspark?