NZ Police have announced the Christchurch court where Tarrant is due to appear will be closed to public over security concerns, but media will still be able to attend.
Two other people arrested are yet to be charged.
“This individual has travelled around the world with sporadic periods of time spent in New Zealand,” Ms Ardern said on Saturday.
“They were not a resident of Christchurch. In fact they were currently based in Dunedin.”
She said inquires were ongoing to establish whether the other two were directly involved.
A fourth person arrested on Friday was a member of the public who was in possession of a firearm, but with the intention of assisting police, she said. They have since been released.
Tarrant, who is yet to be named by police, identified himself online as behind the shooting that began as hundreds of worshippers gathered for prayers in the South Island city on Friday.
Police have said none of those arrested had appeared on the watchlist of New Zealand or Australian security agencies
More to follow