“I was one of more than 30 recipients of the manifesto that was mailed out nine minutes before the attack took place,” Ardern told reporters on Sunday.
“It did not include a location, it did not include specific details,” she said, adding that it was sent to security services within two minutes of receipt.
Ardern said she had read “elements” of the lengthy, meandering and conspiracy-filled far-right “manifesto”.
“The fact that there was an ideological manifesto with extreme views attached to this attack, of course, that is deeply disturbing,” she said.
The death toll in the New Zealand mosque shootings rose to 50 on Sunday when police found another body at one of the mosques, as families waited for authorities to formally identify victims and release their bodies for burial.
Arden said bodies would be handed over to families from Sunday evening.
“It is likely, however, to be a small number to begin with,” she told a media briefing, adding that all should be returned by Wednesday.
Ardern said police would be posted at all mosques while they are open.
The accused gunman, a right-wing extremist and self-confessed white supremacist Brenton Tarrant, documented his radicalisation and two years of preparations in a lengthy, meandering and conspiracy-filled far-right “manifesto”.
Tarrant, 28, was charged with murder on Saturday and was remanded without a plea. He is due back in court on April 5 where police said he was likely to face more charges.
Christchurch terrorist charged with murder post-NZ shootings, no bail requested
Friday’s attack, which Ardern labelled as terrorism, was the worst ever peacetime mass killing in New Zealand and the country had raised its security threat level to the highest.
Footage of the attack on one of the mosques was broadcast live on Facebook, and a “manifesto” denouncing immigrants as “invaders” was also posted online via links to related social media accounts.
The shootings have raised new questions about violence being disseminated online.
Ardern told the briefing that she had been contacted by Facebook operations chief Sheryl Sandberg who had acknowledged what had happened.
“This is an issue that I will look to be discussing directly with Facebook,” Ardern said.
Facebook said on Twitter it had removed 1.5 million videos of the attack in the first 24 hours and it was also removing all edited versions, even those without graphic content.
The violence has also shone a new light on gun control.
Ardern said Tarrant was a licensed gun owner who allegedly used five weapons, including two semi-automatic weapons and two shotguns, which had been modified.
“I can tell you one thing right now, our gun laws will change,” Ardern told reporters on Saturday, saying a ban on semi-automatic weapons would be considered.
New Zealand has in the past tried to tighten firearm laws, but a strong gun lobby and culture of hunting has stymied such efforts.
There are an estimated 1.5 million firearms in New Zealand, which has a population of only 5 million, but it has had low levels of gun violence.