For some reason, the article you linked made up their own (very poor) measure of diversity. Imagine a city that's half white locals, and half white British immigrants (temporary or permanent). Would you consider such a city as diverse? The measure in that news article would put this city as being 50% diverse, even higher than Wellington's 39%.
https://en.wikipedia.org/wiki/Diversity_index
For some reason, the article you linked made up their own (very poor) measure of diversity. Imagine a city that's half white locals, and half white British immigrants (temporary or permanent). Would you consider such a city as diverse? The measure in that news article would put this city as being 50% diverse, even higher than Wellington's 39%.