Two teenage schoolboys were taken to hospital after suffering slash wounds to their necks during a students' production of Sweeney Todd.

The 16-year-olds were taking part in the Stephen Sondheim musical about a murderous barber at their New Zealand private school when their throats were cut with a straight razor blade.

They have now been discharged and are recovering, according to the New Zealand Herald.

St Kentigern College's head teacher Steve Cole told reporters that the razor was a prop which had been wrapped in several layers of cellophane and duct tape, and had been used in "many" previous performances.

"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."

An investigation has now been launched into how the two boys ended up with the sides of their necks cut, though a police spokesman told the Herald it was not a criminal investigation.

The production has been cancelled while the investigation is carried out.

Set in the 1800s, Sweeney Todd tells the story of a barber who, due to a corrupt legal system, is banished from the country by a judge who has designs on his wife.

Fifteen years later, he returns to his old shop in Fleet Street - but this time, he slits his customers throats and disposes of them into the pie shop below.

Times are difficult in London, and the owner of the shop - Nellie Lovett - seizes the opportunity to cash in and sell meat pies to hungry customers.

The play was made into a film in 2007, directed by Tim Burton and starring Johnny Depp as the blood-thirsty barber himself and Helena Bonham Carter as Mrs Lovett.