Corrections is calling for more powers to read, copy and store letters coming in and out of prison after a review into its mail system identified multiple "deficiencies" in the law.
The independent review was ordered in August after the man accused of the Christchurch mosque shootings was allowed to send two letters from his prison cell that should have been blocked.
The botch-up - which came to light after one of the letters was published on the website 4chan - sent Corrections scrambling to strengthen the way prison mail was handled.
In a statement, Corrections chief executive Christine Stevenson said she accepted all 13 of the review's recommendations, many of which were already underway.
"I am confident the changes we are making as a result of the review will reduce the ability for mail sent and received to cause harm or distress, either directly or indirectly, to anyone."
Following the error, Ms Stevenson said she had no confidence in the mail screening system and asked the government for more powers to withhold letters.
MPs last month passed legislation making it explicitly clear that prison managers could withhold all mail which threatened, intimidated or promoted hostility towards any group.
But a review, conducted by Miriam Dean and Grant O'Fee, highlighted further flaws in the Corrections Act and recommended the department seek an "urgent" law change.
Prison staff were not allowed to pass on information from an inmate's mail to the intelligence team even for the purposes of detecting or investigating an offence, the report said.
"Yet such mail may well be relevant to detecting criminal offending in the prison and it makes sense a prison's intelligence unit can have the letter, not just the police.
"Perhaps more importantly, such mail may serve wider intelligence-gathering purposes in protecting both prisoner and public safety - a vital consideration in today's world."
The reviewers recommended the law be amended to give Corrections the power to read, copy, and store mail for intelligence gathering.
It also recommended the department be allowed to copy mail more freely than it can currently.
"As we heard repeatedly from prison staff, decisions about whether to withhold mail often fall into a grey area where there are no easy answers, and it is helpful if a staff member can consult a colleague for a second opinion.
"To obtain such an opinion they must send the original mail itself, or a summary of it, or discuss the matter verbally.
"As one staff member observed: 'What's the difference between reading the original or a copy of the letter?'"
The reviewers also noted that prison staff were currently not allowed to withhold a lot of mail which includes indecent or offensive content, which the public "may well consider should not go beyond prison walls".
"However, prohibiting content based on offensiveness or indecency (as some countries do) may be far from easy to apply and may also risk Corrections becoming, as one interviewee put it, the 'moral police' - a role some staff said they have no wish to take on."
Speaking to RNZ, Ms Stevenson declined to now express full confidence in the mail system, but said it was now far stronger than it was.
"I have more confidence than I did, that's for sure."
She said Corrections was moving as quickly as possible to implement all the recommendations, but she could not guarantee a similar mistake would not happen again.
"With more than 50,000 items of mail leaving our prisons every week, I think guaranteeing would be beyond what I can do.
"What I am confident in is: we have totally strengthened our system."
Corrections Minister Kelvin Davis said he was awaiting advice from officials on what law or regulation changes might be necessary.
"Obviously the [mail] system has been, I believe, poorly run... for a number of years and we're now getting on with making the changes that need to be made."
But National's corrections spokesperson David Bennett told RNZ the review was a smoke-screen designed to protect the Minister and department.
"They didn't do their job. They let that prisoner send out mail when he shouldn't have," Mr Bennett said.
"They should just take responsibility for that and not try to hide behind further changes to make it look like they didn't make a mistake in the first place when they did."
The report recommended that Corrections and MPs give the matters some more thought.
Among the other recommendations were that dedicated mail-monitoring teams be set up, regular audits be carried out, and new manual guidelines be established.