This is a bit of a repost and update for Kubuntu 20.04. Originally I found these dictionaries for KDE neon but they work just as well on Kubuntu 20.04 and should work on Ubuntu 20.04 as well. If you are after a Maori dictionary then there are Maori dictionaries for download at Te Ngutu Kura.
New Zealand English dictionary for Kubuntu 20.04
I found this at FreeOffice in a sox format. Not quite sure what that is but when you extract it it has an en_NZ.aff and en_NZ.dic file inside which are the two files Hunspell needs to use the dictionary. Just copy the files into your /usr/share/hunspell/ directory and then start KDE Settings and under Regional Settings/Spell Check you should be able to choose the New Zealand English Dictionary.
New Zealand English dictionary for Libre Office
This one is a lot easier to get and can be found here. It is pretty up to date and to install it just extract the zip file and then right click on the file and choose open with Libre Office.
New Zealand English dictionary for Thunderbird and Firefox
I am pretty sure I got this one from here but I will stick it here as well. To install it in Thunderbird you need to extract the zip file and then you should be able to right click it and choose open in Thunderbird. If not then try a variation on the install in Firefox method which is to about:addons and then Dictionaries and at the top beside the search box there should be a little cog. Selecting it will drop down a menu where you can choose to install the addon from file.
There is now a new version on the dictionary (two actually) on the Firefox addons site. To install click on your menu on the top right, then click on addons. In the new tab that pops up search for New Zealand under "Find more add-ons" at the top of the page. Or just click the button below which has been updated to take you to the addon page and install it directly.
Also, open source dictionaries are a bit of a bugger to find for Aotearoa/New Zealand sometimes so if you find some useful ones throw them in the comments for me.