National Party MP Alfred Ngaro allegedly punched an atheist teacher at his son's school for not bowing his head during a prayer.

Ngaro, a list MP and former chairman of the Tamaki College Board of Trustees, was last week dragged into the Employment Relations Authority dispute between Tamaki College and former art teacher Christopher Scott Roy.

Roy claims he was constructively dismissed because he is an atheist and Tamaki College saw Christianity as "a core responsibility to which he was indifferent".

Roy added a new allegation to his employment claim, telling ERA member Tania Tetitaha that in 2009 he was assaulted by Ngaro as he was leaving a First XV rugby after-match function at Kings College.

At the time Ngaro was a board of trustees member. He later entered Parliament in 2011 as an Auckland-based list MP.

Kings College officials had asked if anyone objected to a prayer or karakia being said before they ate.

Roy said he did not take part due to his atheism but rather looked around the room as everyone else bowed their head.

Ngaro, whose son was in the Tamaki First XV, came up to him and got "right in my face" after the prayer, Roy told the ERA hearing, eyeballing him just a few centimetres from his face.

Representatives from Kings College saw the behaviour and asked after his well-being, and if he wanted security guards present, Roy said. As he went to leave he was confronted outside by Ngaro, who lashed out at him, punching him on the back of his head.

Members of the First XV broke up the fight, Roy told the hearing.

As he was driving some of the boys home, they told him he was bleeding from the back of his head.

One of the then-Tamaki First XV members, Unaloto Pita, confirmed to the Sunday Star-Times that a scuffle had taken place involving Roy as he left the Kings College function. Pita said he did not see who assaulted the teacher.

Ngaro, appearing in person at the ERA hearing, categorically denied the assault.

Roy said not going to the police was "the worst mistake of my life" but at the time he thought he would jeopardise any future employment opportunities.

The MP, who is married with four children, was part of the negotiations after Roy took the school to the Human Rights Commission over his atheism concerns.

"In any event that he could not participate in, we were happy to accommodate him," Ngaro told the ERA.

The Human Rights Commission complaint arose after Roy chose not to attend a powhiri ceremony held at the beginning of the 2010 school year. He asked at a staff meeting where staff who did not wish to attend the powhiri should congregate.

Another staff member told him: "You're just a f...ing dick" and "keep your f ...ing bullshit to yourself".

Roy said he was later emailed by Principal Soana Pamaka saying attendance at the powhiri was compulsory and "no staff member had any right to be absent".

Pamaka told him the powhiri was cultural rather than religious, though Roy maintained the ceremony had "numerous references to Christianity".

The situation deteriorated after Roy emailed staff at the school calling Pamaka "autocratic" and complaining about his treatment. Roy told the ERA he left the school after being told he could either resign or take a settlement.

He signed a settlement and received a $6500 payout, though he told the ERA he did so under duress and without legal advice.

Tetitaha said before Roy's grievance could be heard, she would have to decide whether the settlement prevented her from hearing it, and whether the fact the grievance was brought outside the 90-day period could be overcome. She reserved her judgment.