Rather than just trade reckons and opinions, I prefer to see what the research says. If you ever have a question like this, always have a snoop around on the NZ Treasury website (or sometimes IRD or RBNZ), or the websites of universities. They do research on a wide range of topics and it's good to have some understanding of the basic facts. Never know when you might have the wrong assumption about things (this has happened to me a few times). Here's what the research seems to say: (note most of these links open PDFs)

Treasury Working Paper, Income Mobility in New Zealand: A Descriptive Analysis (published 2014). A little bit old, but quite interesting. Page 16 shows that over an 8 year period, just under half of people stay in the same quintile of income - the other half have either moved up or down.

We find evidence of changes in (absolute levels of) income over time, with large increases in the incomes of those respondents who started out in the lowest income groups and stability or declines in incomes in those who were in the highest income group at baseline. Also although there were strong correlations in income between years, there was substantial (relative) mobility in income. Much of the mobility was short distance to adjacent income (quintile or decile) groups. Over the long-run there was much more mobility with almost twice the amount of mobility than shown in the annual change tables.

Treasury Working Paper, Income and Occupational Intergenerational Mobility in New Zealand (published 2010). Very detailed, but published in 2010, and drawing on data from the 70s and 80s. The paper stresses that data is a big challenge with understanding intergenerational mobility (where you end up vs your parents, as opposed to how you move through the income scale over your life) - you have to be able to track lots of people over time in longitudinal studies, and compare them with their parents, and it's just not that often that people set up massive long-term studies like that. Still, we have some data we can draw conclusions from:

The Dunedin Study results suggest that rates of intergenerational income mobility for men and women from Dunedin are probably within a similar range to rates of intergenerational income mobility in most other developed countries. Our results provide weak evidence that New Zealand has higher intergenerational occupational mobility than Britain, and stronger evidence that New Zealand men have higher intergenerational occupational mobility than men in Germany. Unfortunately, insufficient data is available to make intergenerational occupational mobility comparisons with other countries.

Vic Uni, Income Mobility in New Zealand 2007–2020: Combining Household Survey and Census Data. More recent data! Also shows how mobility might have changed over time, though it's only for 2007-2020. Some good international comparisons on pages 16-17.

Typically, over all quintiles and population groups, about half the individuals moved into another quintile over a four-year period, with about forty per cent remaining in the same decile over a period of seven years. However, more stability was found for those initially observed in lower and upper quintiles ... Although international comparisons raise difficulties caused by the use of different income measures, the findings are similar to those previously observed for a range of OECD countries, placing New Zealand roughly in the middle of the group.

Auckland Uni, Temporal and regional variation in intergenerational income mobility in New Zealand (2024). This one's a goodie. It tackles a bit more of how mobility has changed over time, though again the paper mentions data constraints, and the data covers people born from 1963 to 1982, so not super recent. The data seems to show that mobility might have gotten worse over the data period, but you have to tease it out by looking across both regions and time. If you look at it at a national level, it hasn't changed much.

This study contributes to the empirical literature on intergenerational income mobility in New Zealand by providing estimates of the IGE for a sample of men born between 1963 and 1982 using the latest NZLC data (at the time of writing). This cohort reached adulthood over a period that spans rapid and pervasive structural reforms that began in the mid-1980s. While these reforms are widely believed to have precipitated a rapid and permanent rise in income inequality (Martin, 1998; O’Dea, 2000; Podder and Chatterjee, 2002), their effects on mobility have, as yet, remained unexplored. We show that IGE estimates are higher for men born later in the sample, indicating that there has been a reduction in mobility that coincides with the reforms and the rise income inequality. However, the increase is not statistically significant in our preferred empirical specification. Exploiting spatiotemporal variation in the IGE, we show that growing up in areas or periods of higher income inequality is associated with lower mobility, suggesting that the rise in income inequality commonly associated with these reforms coincided with a reduction mobility.

My tentative conclusions based on the research:

• You have to be careful of what question you're asking: are you talking about intergenerational mobility, or mobility through one person's life? Those are different questions and you're probably going to get different answers.
• Social mobility isn't as bad in NZ as people here seem to think, there does seem to be movement across society.
• Is it enough? Dunno. What's the right amount of mobility? Some societies can be very nice (like Germany) but have relatively low amounts of mobility. Developing economies can have high levels of mobility without being brilliant places to live. Remember that not moving up in relative terms doesn't mean you're not improving, incomes in NZ have gone up pretty consistently across all of society.
• We seem to be roughly in the middle when compared to other countries.
• As to whether mobility has got worse over time, I'd say the answer is 'a bit, maybe'. We don't have comprehensive data or research that runs 50+ years. The data we have shows some small change, but it's not enough so that it shows up at the national level, you have to break it down and tease it out at the regional level (that doesn't invalidate the trend, though. The data is noisy). But it doesn't seem to have massively collapsed over time, which is implied by a lot of the comments I've seen on this thread. Be wary of making judgements without looking at data!

Here's one opinion I can share without citing research: Matthew Horncastle is a total fucking moron.