"Police are responding with its full capability to manage the situation, but the risk environment remains extremely high," the police commissioner said.
New Zealand police said that all Christchurch schools were in lockdown because of "a serious ongoing firearms incident" that media reports suggested was near a mosque.
Police Commissioner Mike Bush said in a statement that "a serious and evolving situation is occurring in Christchurch with an active shooter." Police did not say where the shooting occurred.
Bush also recommended that residents across the city remain off the streets and indoors.
"Police are responding with its full capability to manage the situation, but the risk environment remains extremely high," Bush said in the statement.
New Zealand police said on Twitter that armed police have been deployed.
Witness Len Peneha said he saw a man dressed in black enter the Masjid Al Noor mosque and then heard dozens of shots, followed by people running from the mosque in terror, the Associated Press reported.
He said he also saw a gunman flee before emergency services arrived. Peneha said he went into the mosque to try and help: "I saw dead people everywhere," he told the Associated Press.
Police said in a statement that officers responded to reports of shots fired in central Christchurch around 1:40 p.m. Friday local time (8:40 p.m. Thursday ET).
Media reported that shots had been fired near a mosque, and a witness told broadcaster One News that he had seen three people on the ground, bleeding outside the building, Reuters reported.
"Horrified to hear of Christchurch mosque shootings. There is never a justification for that sort of hatred," Amy Adams, a member of parliament from Christchurch, tweeted.
Christchurch City Council said in a statement that all council buildings were in complete lockdown, with no one allowed in or out.