Graphic details of the throat slash injuries suffered by two Auckland schoolboys during a production of Sweeney Todd, along with other close shaves and injuries in dress rehearsals, have emerged in a damning WorkSafe report that found the show had "potential for death".

Two Saint Kentigern School boys were taken to hospital on April 6 last year after being seriously injured during the senior school musical production's opening night in front of parents, pupils, and guests.

Just after the start of the second act at 9pm, a student had his throat cut by a covered straight razor when his character was killed by Sweeney Todd, according to the WorkSafe report detailing the show's disastrous opening night.

The victim suffered a laceration stretching 8cm across his neck and going 4.5cm deep. He was operated on at hospital under general anaesthetic and released the next day.

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A WorkSafe health and safety medical practitioner said it was fortunate that the "dangerous laceration" missed the common carotid artery, which if severed "blood is ejected under pressure and saving the person's life in an emergency community situation would be very difficult".

When the throat was accidentally cut, horrified onlookers called out that a second student may also be injured because he was due to be "killed" next.

But teachers, unaware of what was unfolding, couldn't act in time. The second injury happened at 9.01am, with the student actor suffering a 5cm deep neck gash with St John Ambulance paramedics able to see "trachea and some cartilage".

The musical was not stopped and it ran until the end.

WorkSafe was notified of the incidents the next morning at 8.37am.

"The harm involved two victims and to [one pupil] in particular, it was significant. There was a potential for death," the report says.

It also found that the student who played Sweeney Todd had been left "stressed" and suffering from emotional harm.

In the days leading up to opening night, two students at the co-educational Presbyterian secondary school in the Auckland suburb of Pakuranga were also injured, the report also reveals for the first time.

During a rehearsal on April 3, an incident report form says a pupil received a "scratch" that did not bleed. However, the pupil later stated that the cut bled slightly.

The taped blade was allegedly checked afterwards and they "could not replicate the injury".

Also a few days before the show, a pupil received a friction burn from having a covered straight razor run over his throat during rehearsals.

Incident reports appear to have not been passed on.

WorkSafe's report into the incident, obtained by the Herald under the Official Information Act, recommends that the school is prosecuted over the incident.

But despite the decision being backed by the victims' families and signed off by the report writer's manager and chief inspector, WorkSafe last month decided against the idea.

The detailed 17-page report identifies six health and safety failings by the school board.

The director of the play also admitted not giving the teenage actor playing Sweeney Todd any specific advice.

"I did not give any direction to [the actor] as to whether or not the knife blade should touch the 'victim's' skin - this was not a discussion we ever had," the teacher told WorkSafe.

The investigation found that it was unclear how many razors were bought for the production, held in the college's Elliot Hall, or where they were purchased.

One of the four teachers behind the production "used his own initiative" in making the blades safe - covering them in duct tape, tin foil and sellotape, with the person telling investigators they knew they posed a risk so only allowed teachers or actors to use them.

Four knives recovered from St Kentigern were later tested, with two able to cut cleanly through paper and considered "sharp", while the other two were not.

One student told WorkSafe, "Teachers had told students that the blades had been blunted", while another said, "We were told that the razors were real, but that they had been made safe".

The WorkSafe report says said pupils were not consulted about the use of real straight razors, nor were they in a position to challenge their use.

"On balance, the prosecution is warranted," it concluded.

But in spite of the damning report, WorkSafe announced last month the school could use an "enforceable undertaking agreement" instead of facing conviction.

The school instead went through a restorative justice process with victims, including paying them compensation, as well as spending around $80,000 on processes to improve health and safety standards at St Kent's and across New Zealand. It had also apologised to the victims' families.

Saint Kentigern Trust Board today said it accepted the WorkSafe findings.

"We apologise for what happened," the board said in a statement released to the Herald.

"These were very serious injuries and the Trust Board members personally feel deep regret and take full responsibility for the emotional and physical harm that was suffered as a result of them.

"We can confirm that we have commenced a restorative justice process with the families concerned, and for obvious reasons we will not want to comment further about that while this process is underway."

Kentigern College head Steve Cole earlier said that the razor had been "checked many, many times".

"It has been bound and cellophaned and all sorts of things. It had been blunted and had been through all sorts of health and safety checks. It was a very unfortunate mistake."