Witnesses reported hearing 50 shots from his weapons including a semi-automatic shotgun and a rifle at Al Noor Mosque in Christchurch on the country's South Island.
The gunman - who identified himself on Twitter as Brenton Tarrant from Grafton, New South Wales - live-streamed the mass shooting inside the Al Noor Mosque, which happened about 1.30pm (11.30am AEDT) as Friday prayers were underway.
A video seen by Daily Mail Australia shows the man firing multiple shots at dozens of people as they try to flee.
Four people are in custody, including three men and one woman. One of them was arrested while wearing a suicide vest.
The suspected gunman posted a 87-page manifesto to Twitter before the killings, foreshadowing a 'terrorist attack'.
People were also reportedly shot at the nearby Linwood Masjid mosque.
There was another shooting outside Christchurch Hospital and multiple bombs were attached to cars three near the mosque.