Christchurch has a problem with smog during the winter. Although, conditions have improved over the years, due to the severe intervention of the city council, take care venturing out on calm frosty evenings if you have a breathing-related medical condition.
While violent crime is relatively rare, some people do have a tendency towards aggression when drunk, as with most cities. Linwood is considered the 'crime' area of Christchurch, but is still considered safe. As in any city, take care late at night, especially on Friday and Saturdays as levels of intoxication can lead to unwanted attention or unprovoked violence. Avoid dark alleyways and confrontations, and if in doubt, make haste to a populated area and call the police dial 111.
Christchurch witnessed major earthquakes in 2010 and 2011. The city lays in an earthquake-prone area, but aftershocks have been decreasing in numbers and severity as time passess.