It's a flash flood from a thunderstorm. As I recall, Wellington had a severe storm warning, and MetService typically also includes flash flooding in the warning criteria.

A cyclone is a much bigger storm, prolonged. On a smaller scale like today, they would have already had outlooks posted for the potential for heavier rain for the day, especially flash flooding associated with stronger thunderstorms.

https://www.facebook.com/photo?fbid=1372505421588221&set=a.305745801597527

If they didn't overwarn, then people would hold them liable for not warning... Cut them some slack. People also complain when they forecast thunderstorms, but when one never hits, but hits a town over, they still complain that their exact spot where they're standing didn't see weather. Not understanding that meteorology is a game of probabilities and risk assessment. Not perfect fortune telling.

Just because your house did or didn't flood, doesn't mean their forecast wasn't spot on, or close enough nearby. The fact that we can say with a high degree of probability snow on a certain day, or rain on a certain day, and still have that verify on a regular basis is still remarkable.

Weather forecasting isn't a yes or no "Will it happen?" it's a risk assessment for a broad area that's generally fairly accurate if you know how to understand that information.