Correlation doesn't mean causation. Solitude, substance abuse, stress and economic hardships are all known linked causes that tend to drastically increase death rates. Drug overdoses killed nearly 100k people last year alone, not even counting the indirect or unreported deaths

Also you're comparing apple to oranges. Contrary to most Western countries, New-Zealand focused on sealing their country away, meaning they didn't have to have harsh lock downs for a long time.