"We did as much as we could to remove, or seek to have removed, some of the footage that was being circulated in the aftermath of this terrorist attack", she said.
"I have had contact from Sheryl Sandberg [Facebook CEO]".
"Ultimately, though, it has been up to those platforms to facilitate their removal and support their removal".
"Out of respect for the people affected by this tragedy and the concerns of local authorities, we're also removing all edited versions of the video that do not show graphic content", Garlick added.
Ardern said there were "further questions to be answered" by the social media sites.
On Friday, YouTube tweeted it was "working vigilantly to remove any violent footage" while Twitter said it suspended the account of one of the suspects.
Facebook had "further questions to answer" over the live streaming of shootings at two mosques that killed 50 people, as the tech giant said it removed 1.5 million videos globally in the first 24 hours after the attack.
In a statement on Sunday, Mia Garlick of Facebook New Zealand vowed to "work around the clock to remove violating content".
New Zealand Prime Minister Jacinda Ardern speaks with a woman during a visit to the Canterbury Refugee Centre in Christchurch, March 16, 2019.
The shooter also left a 74-page manifesto that he posted on social media under the name Brenton Tarrant, identifying himself as a 28-year-old Australian and white nationalist who was out to avenge attacks in Europe perpetrated by Muslims. "But I sadly have to say that the capacity to actually assist fully is very limited on the technology side". "Clearly it hasn't (happened)".
Hours after the attack, New Zealand police said they were working to have the footage removed while urging people not to share it.
In another case, the video was shared by a verified Instagram user in Indonesia with more than 1.6 million followers.
"So I think there are some very real discussions that have to be had about how these facilities and capabilities as they exist on social media, can continue to be offered".