The man, described as in his 40s and around 2m tall, calmed down only when eight police officers arrived.
He was rushed to Christchurch Hospital for surgery and emergency services say he is in a serious condition. Police said he was becoming increasingly shaky due to the loss of blood when they spoke to him.
He has not yet been charged, although police say they will pursue him when he recovers.
The woman who was assaulted was said to be shaken but unhurt.
"It was very scary, we're all still in a state of shock," said one worker. "It can be a very emotive situation when talking about access to children, and we get some very stressed individuals. We get training for how to deal with aggressive clients, but this was something else."