An Australian citizen has been detained as the key suspect in a deadly terrorist assault Prime Minister Scott Morrison has condemned as a "vicious, murderous attack" on people of faith.

Mr Morrison revealed an investigation has been launched after security agencies told him the man believed to be behind the storming of mosques in the New Zealand city of Christchurch was an Australian-born citizen.

"We are outraged and we stand here and condemn absolutely the attack that occurred today by an extremist, right-wing violent terrorist who has taken the lives, stolen lives, in a vicious, murderous attack that has claimed so many New Zealanders," Mr Morrison said from Kirribilli House in Sydney.

More than 40 people inside and outside the Al Noor Mosque in central Christchurch and the Linwood Mosque in the city's outer suburbs have been killed. New Zealand police have taken four people into custody.