A duct-taped and foam-covered razor blade was the prop that cut two New Zealand student's necks during a performance of the blood-spattering musical Sweeney Todd.

The 16-year-old boys were taken to Auckland Hospital but are expected to be discharged on Thursday after things went wrong on Wednesday night at St Kentigern College's production of Stephen Sondheim's blood-soaked 1979 Broadway musical.

The school's head, Stephen Cole, told One News the boys were wounded on stage during the second act by a real razor that was covered in duct tape, foam and paper.

He said the prop had been checked over many times and didn't have a sharp edge.

Two NZ students hospitalised with neck injuries after Sweeney Todd performance #news pic.twitter.com/MbHSe9smuM — freshnews (@freshnews247) April 7, 2016

"I'm very confident the health and safety situation was strong. It was an unfortunate, isolated incident," he said.

Mr Cole said in hindsight a plastic prop could have been used, but it was important to make the prop as realistic as possible.

Two students hospitalised after their necks are cut during a performance of Sweeney Todd https://t.co/nx8jeNRkY4 pic.twitter.com/ltSMAK3FHz — Daily Mail Online (@MailOnline) April 7, 2016

The play, set in Victorian London, contains scenes where the characters sitting in a barber's chair have their throats slit with a cut-throat razor.

They are then dispatched through a trapdoor to be cut up into filling for meat pies.

Audience members on opening night have reported not seeing anything untoward happen on stage.

The school has postponed Thursday night's performance and says it is reviewing the rest of the week's run.

The incident happened just two days after the introduction of tough new rules came into force over workplace health and safety.

WorkSafe New Zealand says it has been advised and inquiries are being made.

AAP