I think that 2002-2020 period was also probably informed by a lot of older voters who got spooked by the instability of the National-NZ First coalition and it became more desirable to have one really strong party for those voters too.

But there's now a whole generation of voters who weren't alive when Shipley rolled Bolger basically because he was moving slowly to keep his coalition partner on board, only to find that pissing off your coalition partner kinda destabilises your government. I wasn't yet a voter, but that National Party shift was basically from a PM trying to work within the new rules of MMP to one that wanted to act as though National had an outright majority.

I imagine a lot of the frustration of this Labour government is that they didn't do a lot with an actual majority :) But then there's an advantage to being able to pin unpopular policies (that you still really want to do) on your small coalition partner - ie Act pull National governments further right than National parties campaign. But it strikes me that they're quite happy with the policies, just know they can't campaign on them.