Although Adern acted quickly it's primary because they are a) isolated b) have a tiny population in a country the size of the UK and c) it hit during their Summer.They've still had a very impressive low level of infections and deaths.
NZ now have 19 deaths and new cases are starting to increase (no doubt because the country is cooling down as Winter arrives) compared to Hong Kong (4 : no new cases) and Taiwan (6 : no new cases) which have had far less deaths despite being densely populated and on China's doorstep. HK is especially impressive.