A high school musical in New Zealand got a little too real when two boys ended up in hospital after a prop razor used in a throat-slitting scene cut their necks.

Students at Saint Kentigern College in Auckland were performing the opening night of Sweeney Todd for parents and other audience members when things went badly wrong.

Set in Victorian London, the musical depicts a barber who slits his customer's throats with a razor and uses the bodies to make meat pies.

College head Steve Cole told radio station Newstalk ZB the razor used in the show was real but had been bound in duct tape. He said he had no idea what went wrong during a scene midway through the second act.

"It had been bound in Cellophane, bound in all sorts of things," he said. "It was very non-sharp, blunted, and had been through all sorts of health and safety checks. It was a very unfortunate mishap."

He said the boys had been released from hospital and were doing well.

"We've been keeping contact through the family and making sure the kids are okay," he said.

Mr Cole said a performance of the play scheduled for Thursday was cancelled but he hoped the show could resume on Friday - "obviously without those particular props".

Police and health and safety officials are investigating.