Ardern called the shooting a terrorist attack and described it as "well-planned".
Ardern said that New Zealand does not condone racism and is not an enclave for extremism.
Two other people remain in custody, although their link to the attack is unknown.
The gunman's manifesto was a welter of often politically contradictory views, touching on numerous most combustible issues of the day, among them the Second Amendment right to own guns, Muslim immigration, terrorist attacks and the wealthiest 1 per cent.
Worshippers, possibly dead or wounded, lay huddled on the floor of the mosque, the video showed. Reuters was unable to confirm the footage's authenticity and police urged people not to view or share it. The man burst into the mosque as worshippers were kneeling for prayers.
The shooter's two targets were the Masjid al Noor mosque, where 41 people were killed, and a second, smaller mosque in the suburb of Linwood, where seven more died.
He visited Bulgaria from November 9-15 a year ago claiming he wanted "to visit historical sites and study the history of the Balkan country", Bulgaria's public prosecutor Sotir Tsatsarov said, adding that the inquiry would establish if this was "correct or if he had other objectives".
The Quebec City mosque attacked by Bissonnette posted the following brief message: "We are following with worry the terrorist attack that occurred against two mosques in New Zealand".
Tarrant was remanded in custody without plea to the high court in Christchurch on 5 April.
The Herald later quoted one of the witnesses from Lindwood, Syed Mazharuddin, who said that the gunman was confronted by a mosque caretaker, who wrestled one of his guns away but did not shoot because he "couldn't find the trigger".
The 59-year-old Canadian songwriter was due to appear at Hagley Park in the New Zealand city on Sunday, but has since put the concert on hold following the tragic attack.
Police said they have arrested three men and a woman, and while they don't think there are any armed offenders still at large, they can't be sure the danger has passed or the incident is confined to Christchurch.
The online video footage, which appeared to have been captured on a camera strapped to the gunman's head, showed red petrol canisters in the back of his vehicle, along with weapons.
Police warned Muslims across the country not to visit mosques "anywhere in New Zealand" in the wake of the Christchurch attacks.
The visiting Bangladesh cricket team narrowly avoided the shooting after arriving at the mosque for prayers.
Political and Islamic leaders across Asia and the Middle East voiced concern over the targeting of Muslims.