It's not clear what sparked the mass killer's change of heart.
From his maximum-security cell at Paremoremo Prison, the 29-year-old sent formal written instructions to his Auckland-based defence lawyers Shane Tait and Jonathan Hudson that he wanted to plead guilty.
Whangārei-based Cole said the truth may be forever buried under his guilty pleas.
"We and the rest of the grieving families, I am sure, would have ensured that when he went on trial, we'd have asked him why he killed so many people.
"What was his intention and why did he do what he did? That may never come out now that he's pleaded guilty, which came as a surprise," she said.
Cole had planned to attend the trial with her brother and his family in Christchurch and said she may go to Tarrant's sentencing, whenever that happened.
She said her family was still grieving the loss of a loving brother who was well-liked by everyone.
None of the shooting survivors or victims' families, like Cole, knew about Tarrant's court appearance.
Presiding judge Justice Cameron Mander explained that despite Tarrant providing his lawyers with written instructions of his intention to change his plea, there was no guarantee that when the charges were read to him he would do so.
Meanwhile, Northland Muslim Community Charitable Trust trustee Shirley Rankin said Northland's Muslim community is generally relieved there will not be a trial and the killer will not have a platform to spread his hateful message.
''Not just the Muslim community, all New Zealanders will be fantastically relieved by this. This has been a very big weight in our hearts, especially those of us who lost somebody,'' Rankin said.
''With the [March 15] anniversary so many people have been talking to us about it [the trial] and while we really appreciate the empathy of people, it was deeply affecting people.
''But this [guilty pleas] means we won't have to see it every night on the news and we won't have to see the video and the deaths. We are really, really, really relieved nobody has to go through that now.
''But this is a New Zealand issue. This was the whole of New Zealand he attacked, not just one part of it.''
As well, she said, the trial would have cost a lot of money better spent fighting the coronavirus pandemic.
''We are also so pleased that the judge has confirmed that he will not be sentenced until after [the lockdown] so that's a big relief too,'' Rankin said.
Although some people in the Muslim community would not want to go to the sentencing, many would and they need answers.
"What happened to make him so twisted to do that?''
