by Hagar Delest » Sat Jan 20, 2018 1:24 pm
Language settings in documents are not managed by the interface setting.
Since I don't have any New Zealand flavor in the default dictionary delivered with AOO, I guess that you've installed a specific one for New Zealand. If so, it may have no thesaurus. In this case, you can switch the language setting of the text to English US or tweak the NZ dictionary extension so that it uses the English thesaurus (never tried myself but quite sure it's feasible).
LibreOffice 7.0.3 on Xubuntu 20.10 and LibreOffice 6.4.6 (portable) on Windows 10.