New Zealand English Dictionary 1.0.1 for Firefox. Includes New Zealand places, including major cities and towns, and major suburbs. It also contains NZ words, organisations and expressions. Packaged up from OpenOffice's English (New Zealand) dictionary by Tristan Burtenshaw. Installation Instructions

Copy the text, "Kia ora bro. Just paddling my waka down the Waikato river to meet the whanau at the marae. Need some bickies for smoko." into the search for add-ons text box above.
Right click the text box and select Check Spelling to turn on the spell checking.
Notice that the Kiwi words are underlined to indicate that they are misspelt.
Click on the Add to Firefox button above.
Click on the Install button on the Software Installation dialog window.
Restart Firefox.
Change the dictionary to New Zealand English:
1. Right click the search for add-ons text box above and select Check Spelling to turn on the spell checking again.
2. Right click the text box again and select Languages -> English / New Zealand.
If the text above is missing from the text box, copy it again.
All the Kiwi words should now be marked as spelt correctly and not underlined.