Police have reported of a bomb in a auto on Strickland Street and warned people to stay back.
New Zealand historically has had only a handful of mass shootings events.
TVNZ reported that New Zealand Prime Minister Jacinda Ardern has been briefed on the situation, and will address the incident.
"For now my thoughts and I'm sure the thoughts of all New Zealanders are with those who have been affected their families", she said.
Commissioner Bush said every police resource in the Canterbury region had been mobilised.
The head of the Federation of Islamic Federations of New Zealand, Dr Mustafa Farouk, told RNZ that up to 500 people were inside the mosque.
"I've lived next door to this mosque for about five years and the people are great, they're very friendly", he said.
As many as 300 people, possibly more, are believed to have been inside the mosque at the time. The gunman came from the backside.
Gunshots were heard near the Al Noor mosque around 1:40 p.m. local time. "We had to jump the wall to escape", he said. He was driving by the mosque about 1.40pm and saw a whole lot of people running.
"I saw dead people everywhere", Peneha told the Associated Press.
New Zealand media say a shooting has occurred in a second mosque in the city of Christchurch.
He then exited the mosque through the front door - after just under three minutes inside, and headed into the street - firing random shots as cars drove past.
"Police are aware there is extremely distressing footage relating to the incident in Christchurch circulating online", New Zealand Police tweeted.
Christchurch Hospital has urged people to avoid the area unless they are seriously injured. More information will be added as soon as it becomes available.
Armed police have been reportedly checking all vehicles outside Christchurch Hospital amid reports another shooter is in the area.
Police have advised that there may still be other offenders at large and are urging everyone in central Christchurch to stay indoors.
The Christchurch City Council indicated that many government buildings were going into lockdown mode.