UPDATED 7.34PM  The head of St Kentigern College said two boys whose throats were cut in a school show were using a prop razor wrapped in duct tape.

The two 16-year-old boys were hospitalised during a production of Sweeney Todd at the private east Auckland school last night.

They have since been discharged and are recovering at home. 

Stephen Cole has told a media conference the prop "covered in all sorts of duct tape and silver paper" had been used numerous times during rehearsals and shows.

LISTEN ABOVE: Stephen Cole, principal of St Kentigern College talks to Larry Williams 

He said the razor didn't have a sharp edge, and was the kind of razor someone would expect to find in a production of Sweeney Todd.

He told Larry Williams it had been safely used numerous times in rehearsals and shows.

"This clearly had been checked many, many times. It had been bound in cellophane, bound in all sorts of things it was very non-sharp, blunted.

"These particular scenes had been rehearsed, full dress rehearsal six, seven, eight times and this particular scene, many many times since the start of rehearsals".

Mr Cole said he is confident the health and safety situation is strong, and this was an unfortunate isolated incident.

Mr Cole said they've postponed tonight's performance.

He said their focus is on the injured students and their families, and have also offered all students counselling.

Stunt woman and coordinator Dayna Grant said basic oversight could prevent repeats of the accident.

Ms Grant said schools don't need to employ full time production managers.

But she told Larry Williams schools should get a professional to check stunts and equipment, and suggest replicas.

"It could be something as basic as having a knitting needle, and then deciding that they can fall down a trap door with it, not realising that you can get impaled on that knitting needle".

The owner of an antique shop where the school bought the razors said the person who bought them had talked to one of his staff members about the need to cover up the sharp edge.

The man, who asked the Herald not to be named, said he believed the person was a teacher at the school, “a really nice chap,” who said the razors were going to be used in a production of Sweeney Todd.

He could not immediately recall whether the man had mentioned it would be a high school production.

“I know they discussed the safety element and maybe covering up the blade.”

The two blades, which retail for about $40 or so, were bought a couple of months ago the man said.

When the staff member who sold the razors heard what had happened this morning she was quite upset, but the sale of cut throat razors was not unusual and they had been sold to an adult, the man said.

The man said he sold a lot of cut throat razors, especially with the popularity of single blade shaving increasing recently, and said he “never would have thought” this would happen.

“You can sell someone a brick and if they drop it on their foot you would still feel sorry for them.”

Police said they were notified by Auckland City Hospital and there's nothing to indicate anything untoward took place on stage.

A spokesman said it's not a criminal inquiry - and police will be producing a report to help form an Occupational Health and Safety review of the incident.

Katrina Casey from the Ministry of Education said they're offering support to all staff as required, during this difficult time.

Stephen Sondheim's Sweeney Todd is a musical set in 19th century England.

It details the return of barber Sweeney Todd to London after 15 years of exile. Todd slits his customers' throats before dispatching them into the basement through a revolving trapdoor. Their minced-up bodies are later served to shoppers as pies.