Here is how the tragedy unfolded:
-Friday March 15, 1.40pm local time (12.40am GMT): Police respond to reports of shots fired in central Christchurch.
People are urged to stay indoors and report any suspicious behaviour.
Shortly afterwards, all schools in the city are placed into lockdown.
-2.30pm: Police describe it as an “active shooter” situation.
-4pm: New Zealand Police Commissioner Mike Bush says there have been “multiple fatalities” at two locations - both mosques in Christchurch.
He adds that one person has been taken into custody, but warns it is unclear if more than one person is involved, or if any other locations are under threat.
Police urge all mosques across New Zealand to shut their doors and advise people to refrain from visiting them until further notice.
New Zealand’s prime minister Jacinda Ardern calls it “one of New Zealand’s darkest days”.
5.30pm: Mr Bush says three men and one woman are in custody over the shootings.
A short time later, Australian prime minister Scott Morrison confirms one of the people arrested is an Australian citizen.
7.30pm: Ms Ardern says 40 people are believed to have died in the shootings, while more than 20 are seriously injured.
She says the offender is in custody, adding: “I can give that assurance, he has been apprehended. He is also accompanied by two other associates.”
Asked about the attackers not being on intelligence agency watchlists, she says it is an indication they “had not acted in a way that warranted it”.
9pm: Police Commissioner Bush tells a press conference that the death toll has risen to 49 and one person charged with murder.