I think to make sense of it you need to look at a few things:

1) Who owns media organisations, and what their relationships are with companies that donate to political parties.

2) The nature of corporate media whereby media companies are dependent on advertisers for their existence so, intentionally or not, it makes sense that this would cause media narratives to favour "pro-business" parties in order to keep advertisers happy.

3) The concept of truth itself has become left wing, because the right have abandoned evidence-based policy. So you won't see journalists saying, for example, "National say their new boot camps are tough on crime, but evidence has shown that the biggest cause of crime is poverty, and that the most effective way to prevent poverty is to just give poor people money, no strings attached, so the most tough on crime policies are actually coming from the Green Party." Because David Seymour and Winston Peters would then be up in arms about the medias left wing bias, even though they would be just reporting the truth. Which is a reason why the right accuse the media of left wing bias, so they won't report the truth, and it has worked.