The Fire Service in Christchurch is worried someone may soon be killed following more than 30 arsons in the past three weeks.
At least five of the fires have been serious, including one at a rowing club building on the Avon River and several buildings that were close to sleepouts.
Seven fires were lit in Aranui, Wainoni early on Saturday, including two in occupied properties.
Detective Sergeant Chris Power says those areas have a lot of high density housing and, given the time the fires have been lit, there is a significant risk of a homicide.
Mr Power is urging residents to be vigilant and they can expect a quick response from police if they report late-night disturbances.
Police are following strong leads on a suspected link between the fires and some tagging in the area.