A man has been jailed for attacking family members with a replica medieval axe on his wedding night, after he was caught kissing a bridesmaid.

Mark Spencer, 39, was sentenced in the Hamilton District Court on Friday to five years in prison for the February attack.

He pleaded guilty to five charges, including wounding with intent to injure.

Spencer's father, Trevor, who was cut on the face and head in the assault, said yesterday his family had been left shattered.

"It was devastating. It has certainly shattered our family unit," he said.

The rampage began about 11pm on February 13 after Mark and his partner of 18 years, Teresa Mann, were married at their Huntly home. During the evening Mark was seen kissing a bridesmaid and was confronted by his sister, Louise, who told him he would ruin his new wife's day.

He said his wife already knew because they were swingers and in a sexual relationship with the bridesmaid and her husband.

Trevor, 60, said Louise tried to walk away and had no plans to reveal the relationship to anyone, but Mark attacked her. Others intervened to stop the assault and Trevor tried to take his wife, Yvonne, and Louise to the car to leave.

But Mark ran after them and struck his father from behind. "Yvonne went to intervene and he threw her down. A neighbour came up and he threw her down."

Yvonne said Mark returned to the house, took a replica medieval axe from the wall and continued the attack.

"He ran straight past me and whacked his dad in the head with this axe," she said. "Trevor collapsed on the ground and Mark went straight to him and whacked him again."

Louise, who was already injured, stepped in to save her father by wrestling her brother down, saying "you're killing dad, you're killing dad".

Trevor suffered cuts and bruising to his nose and head, needing 20 stitches, and hearing loss in one ear.

Louise received soft tissue damage and bruising.

Although Mark had had mental health issues in the past, family members were baffled by the violent attack, Trevor said.

They have a permanent restraining order against him. Their only contact with him since the attack was a letter in which he claimed to be suffering from post traumatic stress disorder.

Though family members were surprised to learn about his swinger lifestyle, they struggled to understand how being outed could have prompted the attack.

"We've been through counselling. We've racked our brains," Trevor said.

"I can't forgive him for attacking his sister and I can't forgive him for hitting his mum, but he's still my son, and you can't get away from it."

Teresa Mann would not comment. Her parents-in-law said she was struggling to accept what had happened. Their main concern was the welfare of the couple's two young daughters.