Troy Mercanti, 45, is facing five charges, including grievous bodily harm and assault against Tammy Kingdon, his partner of 15 years and mother of his two sons.
He has pleaded not guilty to all charges.
Michael James Mercanti was today called by the prosecution to give evidence about what he saw at the family Christmas gathering at their parents’ house in 2006.
Ms Kingdon alleges that Mr Mercanti hit her in the mouth, knocking out one of her teeth.
Mercanti’s counsel Colin Lovitt QC has argued that she fell down the stairs to the garage – a story which was told to Royal Perth Hospital emergency staff at the time.
Michael Mercanti said he had been at the party for about an hour and was at the back of the house when he heard a commotion.
He said he rushed to the front of the house, worried that his mother might have fallen on the tiles.
When he came into the hallway, he saw Ms Kingdon with a bleeding face.
“I saw my mother pushing my brother away in the chest,” Mr Mercanti said.
“And Tammy was holding her mouth.”
He said Ms Kingdon’s face sons had come from watching television with their cousins and started “punching and kicking” their father Troy Mercanti to distract him from their mother.
Michael Mercanti said Ms Kingdon was yelling “he’s knocked my teeth out” and he ushered the children away.
When he returned to the room, Ms Kingdon had already left for the hospital with his brother Jason Mercanti and he started to clean up by rolling up the blood-stained floor rug.
Under cross examination, Mr Mercanti admitted there was some pressure to make a statement when he was approached by police because he did not want to talk about the day, but police said they would approach his wife if he did not speak to them.
He told Mr Lovitt it “was a terrible day for my family” but he had never assumed his brother had hit Ms Kingdon and conceded she could have fallen down the stairs because he only saw the aftermath.
Doctors from Royal Perth Hospital gave evidence today, including the plastic surgeon who repaired Ms Kingdon’s eye socket when it was fractured in 1997.
The accused's mother is expected to be called as a prosecution witness tomorrow along with police officers.