"I was walking past and I fell, tripped and landed in the corner of his desk, which is quite high, and landed right here, right on my bad hip," she said.
"In that one, two seconds, I did something very, very, very serious and broke that bond that we had . . . the pain was excruciating. I think I just went into trauma mode.
"I don't know what else to say. It was terrible. Absolutely terrible. It's a teacher's worst nightmare."
I'm struggling a bit to understand how it could have been light hearted if you were in such pain.
In her frustration at having tripped on some of the boy's items on the floor, and in a reaction to the pain she was in, the teacher grabbed a laptop case made out of something similar to wetsuit material, and hit the boy three or four times across the head with it, making him cry.
The teacher told tribunal members her actions were worse because the boy had a disorder and was seeing a clinical psychologist to address his own temper issues and "major outbursts", and she had just done what he was being taught not to do.
She said hitting him with the cover was supposed to be "light-hearted" to "get a point across", but once she saw the impact it had on the boy she realised how serious it was.
Complaints Assessment Committee lawyer Abigail Van Echten did not believe the description matched up with the teacher's actions.
"I'm struggling a bit to understand how it could have been light-hearted if you were in such pain," she said.
She pointed to an email the teacher sent to the boy's parents describing the incident, where she said she might have treated him "harshly".
The parents of the boy, who was also regularly bullied, described him as "traumatised", Van Echten said.
The teacher said the boy was "absolutely devastated" when she hit him, but believed he moved on quickly and saw no indication later that he was traumatised.
She said he has now moved on to Year 8 and is in another class, but still comes to see her.
The teacher has since started taking homoeopathic stress remedies and goes to yoga to help with her stress management.
Colleagues praised her as a teacher, with one saying whenever she walked into her classroom there was an atmosphere of "vibrancy and enthusiasm".
The teacher's lawyer, Janette Andrews, said the matter was an "isolated event".
The tribunal's decision will not be made yet, but members spoke of a possible mentoring programme for the teacher.