Christchurch lies on a river plain and most of the time in winter it's not that windy, so the smog settles pretty quickly. Add to that all the fireplaces and woodburners in people's homes than you have a recipe for bad air. It's supposed to have been worse 10-20 years back, but I think it's still bad. So bad that when I went to Wellington on a short trip I noticed after having cleansed my senses with fresh windy sea air, that all my clothing, my bags and myself smelled like "soot" as if I'd been lying smouldering in a fireplace. Yuk!
Ok, it's neither a warning nor a danger, but I've got to post this somewhere, and this is one of the few categories that has a website link built into it.
This site mentioned below, by the Christchurch City Council gives you up-to-date weather info. Very useful.