The prop which cut two students in a Saint Kentigern College production of Sweeney Todd was a real razor wrapped in duct tape and paper, the school has said.

The two students hurt during a throat-slitting scene in the performance of the musical Sweeney Todd on Wednesday night are in a stable condition and are likely to be discharged from hospital on Thursday, principal Steve Cole said in a media statement.

Cole described said the razor used in the show had been filed down and described it as "authentic" which was "covered in all sorts of duct tape, foam and silver paper", reported One News.

SUPPLIED Johnny Depp as Sweeney Todd in the 2007 film adaptation by Tim Burton.

He said the drama teachers involved were "struggling", but were "very competent and have run numerous shows at the college".

He said the prop had been used throughout rehearsals.

PETER MEECHAM / FAIRFAX NZ St Kentigern College in Pakuranga, Auckland. Two students were injured in a production of Sweeney Todd.

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Opening night for the Saint Kentigern's production was Wednesday, but it didn't end well.

"It was an unfortunate, isolated incident which we are very disturbed about," he said.

Cole did not believe there would be any ramifications for the teachers. But the school will undergo an investigation by Worksafe New Zealand .

Counties-Manukau police visited the school on Thursday and made "initial enquires" about the incident, a police spokeswoman said.

The exclusive east Auckland school has also launched its own investigation into how the students suffered injuries to their necks during Wednesday's opening night performance of Sweeney Todd.

Cole said he was "very concerned" about the accident.

One 16-year-old student sustained serious injuries and another had moderate injuries. Both were taken to Auckland Hospital Wednesday night.

Cole said Thursday night's performance would not go ahead while an investigation was under way to determine exactly what happened and ensure it did not recur. However the play was likely to be staged again on other nights.

"Our priority is to continue to support the boys and their families, and we would appreciate if their privacy is respected.

"We are also very focused on helping other cast and crew members, along with the wider school community and have offered all students counselling."

An eyewitness said the audience on Wednesday night had no idea an accident had happened and that the blood spilt was real blood - not all fake blood.

"No announcements were made to the audience that the throat slitting was not all just 'fake blood', the show went on, we never knew anything about the real blood being spilt until later," the audience member said.

The woman, who did not want her name used, said it was a terrible accident.

"It is terribly sad what has happened. It's a great show and I'd hate to see anything bad overshadow the quality of the show."

She said she hoped the boys would recover quickly and "the show would go on".

Penny Fitt, Associate Director at Toi Whakaari said it was not normal practice in productions to use real knives and if so, they are usually made to be very blunt.

In some versions of the musical, adapted from a Victorian penny dreadful, Sweeney Todd (a barber) slits his victims' throats before disposing of them through a trapdoor.

According to the school's website, the St Kentigern's performance tells how Sweeney Todd, a barber turned serial-killer, sets up a barber's shop above a struggling pie shop and kills his victims using a razor blade.

He and Mrs Lovett, the pie shop owner, then turn the victims' corpses in to meat pies.

The college website said; "The biggest challenge of all, however, was the ghoulish murder scenes themselves, as Todd's barbering skills cleanly dispatched victim after victim. Let's just say there was 'gore' – and plenty of it!"

The show was scheduled to run until Saturday, April 9 at the school's Elliot Hall.

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