One of the features of our Mixed Member Proportional system is that it virtually guarantees permanent minority government. Our political landscape makes parties unlikely to gain an outright majority, and memories of the 80's blitzkrieg mean that voters will abandon a party if it looks likely to be able to govern alone (as they did to Labour during the 2002 campaign). Instead, we prefer governments which have to depend on others in order to pass legislation, viewing this as an essential safeguard to prevent 80's-style radical reform from happening again.
This inevitability of coalition means an inevitability of compromise. Parties' election manifestos aren't so much promises of what they will do as what they would like to do, if they can get the support. The acceptance (or at least compatibility) of a party's policy position with that of its allies is thus crucial when assessing whether a party can actually implement what it promises. And here, we have a very clear difference between the parties: Labour's policies enjoy wide support among their prospective coalition partners, while National struggles to get agreement, or even to find partners at all. Their key policy - tax cuts for the rich - has been rejected by Winston Peters and rejected by United Future. The only possible partner who supports their programme is ACT - and they are unlikely to even be in Parliament. The same holds with their Maori policy - while there is more agreement, a core component, the abolition of the Maori seats, has likewise been rejected by their potential partners. Their plans for the Families Commission do not meet with the approval of Peter Dunne, while future privatisations (of LandCorp and the state-owned electricity companies) are opposed by New Zealand First. National's response has been to declare these policies "bottom lines" and insist that any partner must sign up to their complete policy platform. But this has the effect more of ruling them out of power than ruling them in.
Contrast this with Labour's position. While its key policies - zero interest on student loans, an extension of working for families - have been criticised by their preferred partners, the criticism has been that these moves do not go far enough, and there's no question of Labour being able to get the support of the Progressives and Greens to pass them. And in the event that the LPG bloc does not get a majority, the other parties have not shown the outright opposition to these policies that they have shown towards National's. The upshot is that Labour can deliver on its promises. Oh, there will be compromise, tweaking, and horsetrading, because that's what happens under MMP - but broadly, they will be able to deliver. National can't - and that holds even if the centre-right bloc gains a majority. There just isn't the necesary agreement and consensus - or the goodwill - on that side of the political spectrum for National to advance their programme. And their inability to compromise and insistence that every policy is a "bottom line" simply makes getting the necessary agreement harder, not easier.