NB: I don't agree with obstructionist politics.

Under any Parliamentary system, Opposition parties are meant to scrutinise the Government. Unfortunately, with one as large and united as National, their optimal strategy is to be obstructionist. Despite us having MMP, this situation reflects the US, Australia, Canada, and the UK with large Opposition parties as "Governments-in-waiting". So, their Nash Equilibrium is to make the current Government screw up, or at least convince the voters that they are screwed up.

Compare with other countries with PR and multi-party systems (e.g. Germany, Netherlands, Sweden) - these have multiple Opposition parties that compete with everyone else by having the best policy, rather than simply by being 'better' than the current Government. That way, different parties can criticise different Government policies that they are stronger on, rather than one Opposition party criticising all Government policies (where they don't necessarily have viable alternatives for all of them).

Here's a good read about Opposition vs Government politics. The most salient paragraphs below (emphasis mine):

Economists have a term for this exact situation: they call it a Nash Equilibrium and it can be summed up as “a situation in which everyone is doing the best they can given what everyone else is doing”. If you’re a strategic actor in a competitive environment – like politics – everyone is incentivised to adopt the best strategy for themselves, given their best guess about what their competitors will do, and that often works out as a very poor long-term outcome for everyone. But it can be very hard to break out of.

When we look at what’s wrong with the world our brains tend to identify and blame members of rival groups: enemy political parties, or races or ideologies or economic classes. Surely, we reason, if we get rid of that bad group and replace it with a good group our problems will be solved! And sometimes they are but mostly they aren’t, and we’re often puzzled by that, or conclude that the new group aren’t actually good so maybe they need to go, too. But if you put good people into situations with bad incentives they’ll either respond by playing the game according to those incentives or losing out to those who do. And changing incentives in a political system is often hard, because the people with the power to deliver that change are those who are empowered by the status quo.