A New Zealand school production of Sweeney Todd was called off on opening night after two teenage cast members’ necks were cut with a prop razor.

Ambulance services were called to Saint Kentigern college in east Auckland just after 9pm on Wednesday night. Two 16-year-old boys were taken to Auckland hospital, one with serious and one with moderate injuries.

Both were acting in the school’s production of Sweeney Todd, Stephen Sondheim’s 1979 musical about “the demon barber of Fleet Street”, and were injured halfway through the second act about in a throat-cutting scene.

Fairfax New Zealand quoted an audience member who said she did not know there had been an accident until after the show.

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

“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.”

Counties-Manukau police visited the school on Thursday, though a spokesman told the New Zealand Herald it had not been “not a criminal injury”. “In this instance the play is not the reality,” he said.

Worksafe New Zealand, the government service responsible for workplace health and safety, is investigating.

In a radio interview on Thursday, the Saint Kentigern college head, Steve Cole, said the prop razor had been blunted with a number of materials, including duct tape and foam, and the injuries were the result of a “very unfortunate mistake”.

“It’s normal for Sweeney Todd to have such an instrument [and it] clearly had been checked many, many times,” he said.

“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.”

Cole said he had “no idea” how the boys had come to be injured. He understood that both had been discharged from hospital.

He said the play had been rehearsed since January, and that particular scene had been practised “many, many times”.

Thursday’s performance would be postponed but Cole said he hoped the season could be rescued – “obviously, without those particular props”. “I’m very, very hopeful that we will have a full production Friday, Saturday and maybe Sunday, a matinee.”

Cole told other media that, “in hindsight”, it might have been better to use a plastic prop.

A post to Saint Kentigern’s website on Tuesday, before opening night, said the choice of Sweeney Todd had “raised the bar and challenged our student actors, musicians and stage crew on many levels”.

“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!”