He was arrested 36 minutes after the first call to police, Ardern added.
A police officer looks for explosive devices near Masjid Al Noor mosque in Christchurch, New Zealand, March 17, 2019.
He added that the country was proud of Naeem Rashid, one of the victims, who the premier said would be recognised for his courage with a national award.
Police say they will continue guarding mosques around the country until further notice.
"As soon as New Zealanders hear that someone was legally able to acquire, as I'm advised, those weapons and carry out this event, that will raise enormous questions with our gun laws, and that is why we will respond swiftly", she explained.
Two other people were in custody and police said they were seeking to understand whether they were involved in any way.
Ms Ardern confirmed her office was one of about 30 that had been sent the manifesto nine minutes before last Friday's massacre, but it did not contain details of where the attacks would take place.
The mosque attacks have shaken this usually peaceful country, which prides itself on welcoming refugees fleeing violence or persecution.
"The magnitude of this is the thing that is the most significant issue for people".
She said neither the gunmen nor the suspected accomplices were on any terrorist watchlist in New Zealand or Australia.
Director-General of the Pacific Community (SPC) Colin Tukuitonga also showed his support for New Zealand and those who lost loved ones.
The grandmother of the man allegedly responsible for the twin Christchurch mosque terror attacks that have killed 50 has spoken to 9News of her disbelief and devastation.
New Zealand has in the past tried to tighten firearm laws, but a strong gun lobby and culture of hunting has stymied such efforts. But already, the stories of victims from across the Muslim world came into focus.
New Zealand officials said at least one body would be returned on Sunday night, and that all 50 should be back with their families by Wednesday. "Why not release those who have been identified?" he said.
At Christchurch's "Cardboard Cathedral" - built after the 2011 earthquakes that still scar this close-knit city - Dean Lawrence Kimberley held a service to stand "in solidarity with the Muslim community".
Moreover, Pakistan's High Commissioner and Deputy High Commissioner in New Zealand would also be available for guidance, round the clock, the press release said.
"Across all religions, our houses of worship are a source of refuge, of prayer, and of love; to see such a heinous and hate-filled act occur in what should be places of peace is the darkest of evils".
Pence also affirmed USA cooperation in ensuring all the perpetrators were brought to justice.