Copper wire thieves are wearing hi-vis vests to look like legitimate lines workers in Christchurch, as repeated vandalism costs city businesses thousands.

Police have charged seven people with stealing copper wire from power lines near schools, preschools and residential streets.

The thefts began last year in the suburban red zone but have since spread to areas where the electricity is live.

He said thieves had left live powerlines hanging and cost the business thousands of dollars in fees.