Emergency services were called to a house in Raumanga on the night of August 22, 2019.
The baby died at the scene.
The mother told the court yesterday she would take her baby away from her partner when the latter become "rough" and the five-month-old would cry.
She claims she saw her partner squeezing the baby with his knuckles on both sides of his ribs and on top of his head, slapped him on his forehead and mainly around the back of his head.
On one occasion, she said the baby's father held the baby then dropped him onto the carpet in the lounge which resulted in the child fainting.
"I then came to the realisation that he tried to strangle him. I saw his face change so I picked him up, held him in my arms, walked towards the kitchen, and tried to massage him."
She then returned to her partner and asked him what he was doing.
"He burst out laughing and said, 'Don't worry, all I am trying to do is to massage him to make him strong'."
The mother said her partner also threw him up and caught him and held him by the underarm and swung him between the legs.
Initially, she said, the child looked happy but there came a point when he became scared of what his father was doing to him and started crying.
The couple met while working together in Australia before moving to Auckland and then to Whangārei.
The trial continues.