To win an election in New Zealand you have to win in Auckland and Labour simply didn't do enough winning in the City of Sails. As I wrote last night, Labour's campaign failed completely in West and South Auckland, and only performed well - growing its party vote at or above its average nationally in six out of the 22 Auckland electorates - all electorates that you'd typically define as relatively urban and/or affluent.
Conversely, of the 12 electorates where National actually grew its party vote across the country on Saturday night, they run in a corridor starting in Upper Harbour and running through West and into South Auckland.
Where Labour did particularly well is in Mt Albert and Auckland Central which almost certainly has to do with Jacinda Ardern. In Mt Albert the result will have been bouyed by her being the local MP as well as Labour Party leader, and voters generally tend to reward that (though not always). In Auckland Central I suspect what we're witnessing is similar to what was seen in Wellington Central, where there wasn't so much as a youthquake, so much as a youth seismic swing, where youth voters (defined as those 34 and under), and especially students, have switched their votes from National and particularly the Green Party, in behind Labour.
Once we get special votes in and enrolment numbers for those electorates, it'll be interesting to see how it changes, and whether there was increased enrolment and turnout in student heavy electorates, or just a wave of students switching their votes between parties.