Because the polls are showing a Labour, Greens, Progressives, Maori alliance could govern, National is crying that the largest party has some kind of ‘moral mandate’ to govern. Much as the cricket team which has the highest individual run scorer has the moral mandate to win the game, I guess. Back in the real world, though, it is common for governments in countries that use proportional representation to not include the largest party.
In the last 12 elections in the Netherlands, 3 have not included the largest party (the left-wing Workers’ Party)
The Social Democrats are the largest party in the Swedish Parliament but the right-wing Alliance for Sweden, led by the Moderate Party, governs. This was such an unexceptional event in Sweden that the Moderates and their allies declared victory within three hours of the results coming in and the leader of the Social Democrats resigned at the same time with nary a whimper about moral mandates.
In Germany, right-wing coalitions governed despite the Social Democrats being the largest party in 1949, 1965, 1982, 1983, 1987, and 1994.
In Austria in 2002, the major right-wing party won the most votes but the Social Democrats governed with the far-right party.
I could go on, but the point is this: the will of the people is what matters in a democracy and when the people have voted for a bloc of parties that can work together and command the confidence of Parliament that bloc is the legitimate government. National would have no moral mandate to govern when a majority of people had voted for parties that oppose, or are expected to oppose, National.