Prime Minister Jacinda Ardern acknowledged today there was a "systematic failure" after a letter from the Christchurch terrorist attack accused surfaced online. But National characterised it as more than a "one-off mistake".
"It is incompetence," said National's corrections spokesperson David Bennett.
Yesterday, it was revealed the accused gunman sent a letter from Auckland Prison at Paremoremo that was later posted to the message board 4chan.
"I understand corrections have issued an apology," Ms Ardern said today. "No one wants extra trauma for the victims, extra trauma for Christchurch and no one wants this alleged offender to spread his message of hate in the way he's been able to.
"Now we have to do everything we can to prevent this kind of thing happening again."
Ms Ardern said it was clear there had been a systematic failure.
"We have to make sure the department is going back and checking their systems and equally, that we're checking the law.
"If there are any holes here that need to be fixed, that needs to be rectified immediately."
Today on TVNZ1's Breakfast, Corrections Minister Kelvin Davis said the accused sent seven letters from prison. Mr Davis said he didn't know who the recipients were for five of them.
Mr Davis also apologised to victims of the terrorist attack, adding "the situation isn't good enough".
"I say I'm sorry - this should never have happened," he said. "We know that it has caused immense distress, we know that people are really feeling because of this."
But that's not enough, suggested Mr Bennett, the National spokesperson.
He said Mr Davis "should have questioned his officials as soon as he was locked up about what procedures they had in place and been kept regularly updated".
"There are already laws in place which should have been used to stop this from happening," he argued.
Under the Corrections Act 2004, sending mail is a legislatively required minimum entitlement for prisoners. However, mail can be withheld in a very limited number of circumstances.