Need to update your portal address at myir.ird.govt.nz?
You can add your private box as a mailing address by clicking “I want to…” and going into “Manage names and addresses” where you can change your “Default postal address”.
For Auckland
If you have a Suite number, put it into the “Attention” or “Building” fields. IRD also likes the Unit A being specified seperately like so:

The output should look like this (but with your own unique Suite number):

For Wellington:
You can then add your mailbox number to the “building” field. Like this:

You can then verify your address:

Once verified, you can submit the address to update your postal address with IRD.
Any questions? Contact our support team.