'An Australian in custody'
On Friday, Prime Minister Scott Morrison confirmed the Australian was in custody over the attacks.
"Australian authorities are involved and they will be proceeding with their investigation," he said.
Earlier, a Facebook Live video was posted by the account "Brenton Tarrant 9" with the credit "Brenton Tarrant was live" which showed the gunman attacking Christchurch's Al Noor mosque.
In a 74-page manifesto setting out the reasons for the attack, reportedly authored by the shooter, he further described himself as "just an ordinary white man, 28 years old. Born in Australia to a working class, low income family".
In the manifesto, seen by SBS News, Brenton Tarrant said he "only arrived in New Zealand to live temporarily whilst I planned and trained" but then decided to carry out an attack there.
It is unclear how long Tarrant has lived in New Zealand for, nor his connection with the other suspects. He did mention that he made some money from Bitconnect, which is a cryptocurrency like Bitcoin. He then used the money to travel across Europe.
"This is an individual who at the time of the attacks had based himself in Dunedin," explained New Zealand Prime Minister Jacinda Ardern on Saturday.
"But I would describe them as - him as someone who sporadically visited New Zealand but spent periods of time here on the occasions that he visited."
Under a heading "why did you carry out the attack?" he says it was to avenge "thousands of deaths caused by foreign invaders."
Former intelligence and defence policy analyst Dr Paul G Buchanan said the shooting was "a classic case of right-wing extremism, right-wing terrorism."
He told SBS News that the manifesto reminded him of Norweigan terrorist Anders Behring Breivik.
New Zealand Police Commissioner Mike Bush confirmed on Friday afternoon that police had arrested four people, three men and one woman, but said he wouldn't "assume that the threat is over".
He added that he was not aware of other people being involved, but said: "we cannot assume there are not others at large".
Mr Bush also confirmed that reports of improvised explosive devices being strapped to vehicles had been deemed safe by defence force personnel.
According to the New Zealand Herald, between nine and 27 people have been killed.
New Zealand Prime Minister Jacinda Ardern, speaking from New Plymouth, said it was "one of New Zealand's darkest days", describing the incidents as "an unprecedented act of violence".
Additional reporting: Maani Truu, AAP