Prime Minister Jacinda Ardern said 10 people were killed at Linwood and 30 at the Deans Ave mosque near Hagley Park. A further 20 people were injured.
Ardern said it was an "unprecedented" situation and described it as a terrorist attack.
Those in custody had not been on any terrorism watch list: "It's not a matter of someone having slipped under the radar."
She said it was obvious the attacks had been planned for some time. Three people were arrested in relation to the attacks, while one other was arrested separately.
Ardern said she could not give any more detail about the alleged offenders.
There were two bombs attached to suspects' vehicles but they had been disarmed.
She said she was asking questions about them, including access to firearms, of her officials.
The National Security Threat Level had been lifted from low to high.
