In Christchurch, the areas with the worst reputations would be Hornby, Templeton, Aranui, Shirley, New Brighton, Linwood, Hilmorton, Hoon Hay and the areas surrounding them. I wouldn't recommend living too close to the city because it's full of boy racers (car hoons) etc. Riccarton/Ilam is full of idiot students who are drunken vandals so you might wanna avoid the areas around the university too.
But then again, lots of people live in these places and probably find them delightful.
Like the other guy said, the best way to tell is to look at the neighbourhood, (which of course you can't do if you're looking over the internet) but don't fall for Hoon Hay/Hilmorton. It actually looks quite nice, but I know a few people who have been beaten up there.
I assume you're not looking for somewhere ultra-rich if its a temporary thing. Maybe look into suburbs like Burnside, Bryndwr, St Albans, Spreydon, Beckenham, Northcote if you're wanting something average. As a general rule, northern suburbs are nicer & eastern suburbs are nasty.
Oh, and ignore what that other guy said about closer to the water (sea?) being better. Not true at all. Eastern suburbs nasty... The richest suburbs are slightly north of centre city and up on the hills.