'Zealander' originates from English, specifically the word 'New Zealand', which in turn comes from Dutch 'Nieuw Zeeland', where 'nieuw' meant 'new' and 'Zeeland' meant 'sea land'.

'Nieuw Zeeland' (Dutch) was applied to the islands charted by Abel Tasman in the 17th century; it became 'New Zealand' in English, and the agent noun 'Zealander' developed to refer to its inhabitants.

Initially it could refer to someone from the Dutch province 'Zeeland' (or places named after it), but over time it evolved to refer primarily to 'a person from New Zealand'.