Has the left actually been resurgent?
Labour has definitely, but primarily at the expense of the Greens and possibly NZ First based on Party vote. They have taken the three seats of Dunne and the Maori Party and it is the loss of the Maori party that has stopped National from being able to form a Govt.
Depending on special votes, which tend to be more left, National will be down a couple of percent, Conservatives are down a couple of percent, and Act down half a percent. So that looks to be a 5 point swing away from right parties.