Quite a bit more of that kind of thing expected with the increase in extreme weather events from climate change. In Westland alone there are another 12 at-risk old landfill sites. It's estimated that the cost of preventing further spills from the one already-open landfill alone is ~2.3M. In an economically depressed region like Westland it won't be funded, and they're far from the only region in NZ that's facing this problem. Just the first to have experienced a spill event.
Unfortunately the purely-for-profit structure of these long-term ventures can mean that there is no money to cover the cleanup if things go really wrong, depending on who owns the site. I think you need to look at some sort of cleanup fund in escrow as a precondition for building sites like this, which should incentivize improvements in safety measures too.
For reference this was the discussion that happened around it on HN then https://news.ycombinator.com/item?id=20433851
It’s a different issue in countries with less land (Japan being the most extreme one) where landfill is just too expensive because land is expensive. But the US has lots of unused and cheap land that nobody would notice if you added a well-run landfill to.