A weird new poll came out showing National can now form a government. It does seem support may have shifted against Labour, and Simon Bridges isn't doing as badly as he was a few months ago. Here are the results, brackets show the change from 2017.
National-46%(+2%)
Labour-39%(+2%)
Green-7%(+1%)
NZ First-4%(-3%)
ACT-2%(+1%)
National and ACT combined would have 61 seats, a bare majority, with wasted votes from NZ First falling below the 5% threshold being the deciding factor. This is after the NZ First donations scandal, but before the Labour conference a few days ago where Labour promised a significant investment in school upgrades.
Honestly I don't think National will win the next election. I don't want to just dismiss polls, but I don't think this is how New Zealand will actually vote. NZ First I think could easily end up above the 5% threshold despite bad polls as they usually outperform the polls and their support usually increases in election campaigns (2017 being a notable exception due to the outbreak of Jacindamania). Simon Bridges is still a lot less popular than Jacinda Ardern, though her ratings have fallen slightly in this poll.
I think approval ratings of leaders is a key. As long as Ardern's approval ratings are positive seems unlikely she will lose while if opposition is negative that makes little sense. Whenever a leader loses, usually their approval rating is in negative territory and usually their opponent is either more popular or at least fairly close. My understanding and I could be wrong is there are more New Zealanders who lean National than Labour so with no election on the horizon, people just saying what they usually vote.
I remember in Canada going into 2008 election, most polls showed Liberals tied or slightly ahead of Conservatives, but on approval rating, Stephane Dion (Liberal leader) had half the approval rating Stephen Harper (Conservative PM did) and once campaign started numbers moved towards that.
That is not to say National couldn't win, just that from my experience at least in other English speaking countries, leaders who get defeated usually have much worse approval ratings. If you look at G7 leaders, I believe every single one has a negative rating yet if an election were held today, probably asides Italy, all of them would be re-elected.