Robert Samuel Geeves threatened to throw a chair through a window after his son refused to accept Amber Haigh's newborn baby as his brother, a court has heard.  

The disappearance of Ms Haigh, who was 19 when she was last seen in June 2002, is the focus of a murder trial unfolding in the Supreme Court in Wagga Wagga.

Prosecutors allege married couple Robert Samuel Geeves and Anne Margaret Geeves, both 64, killed the teenager after encountering hurdles in their plot to gain custody of her baby.

The Geeveses have pleaded not guilty and insist they do not know what happened to the girl after they dropped her off at the Campbelltown railway station.

Ms Haigh had a five-month-old son with Robert Geeves and had been living on-and-off with the older couple at their property near Young in regional New South Wales for about two years.

Robbie Geeves, the Geeveses' son, appeared as a witness during the trial on Wednesday.

Ms Haigh moved into Geeves home, despite son's protests

Robbie Geeves told the court that Ms Haigh, who was just two days younger than him, became his girlfriend while they were both studying at the Young TAFE in 1998.

The teenage girl moved to Mt Isa to live with her aunt later that year and the couple decided to end their relationship about a year later.

"I spoke to her occasionally on the home phone … and [my father] sometimes spoke to her as well," Robbie Geeves said.

Robbie Geeves began dating another woman in March 2000, but about two months later, his mother told him his ex-girlfriend, Ms Haigh, would be coming to live at their farm.

He told the court that he pushed back against the idea, but his mother told him that "doesn't matter" and "she'll move in because your dad wants her to".

Robbie Geeves said he quickly moved out of the house to a property in Young with his partner Natasha Cross.

Nearly two years later, he saw Ms Haigh walking through the town while "heavily pregnant".

Ms Geeves spoke about having more children 'fairly often'

Robbie Geeves said his mother, who had multiple miscarriages and a stillbirth, talked about wanting more children "fairly often" during his childhood.

He told the court that in early 2002, she appeared at his front door and asked him to "come in the car" so he could see "a surprise".

She was allegedly "quite happy" as she drove him to the Young Hospital, where she told him his father and his new "baby brother" were inside.

"I told her to take me home," Robbie Geeves told the court.

"It wasn't right. You can't have a baby brother to your ex-girlfriend."

Robbie Geeves said he was eventually brought home, but the incident made him "physically sick" and eventually caused him to resign from his job at the vineyard where his parents also worked.

Accused killer likened teen to a 'surrogate mother', court hears

The court heard from Robbie Geeves and Natasha Cross about a heated confrontation that occurred about two weeks later when the Geeveses brought the baby boy to their front door.

Robbie Geeves said his parents urged him to "look at and hold" the baby, which he repeatedly refused.

Ms Cross said the incident was "heated" with lots of "verbal abuse" from the older couple.

During the conversation, Ms Geeves allegedly asked the pair: "Haven't you ever heard of a surrogate mother?"

He said while his parents were leaving the house, his father threatened to "come back later and throw the chair through my front window".

The court heard that Robbie Geeves got an apprehended violence order against his father that night due to the threat.

Teenager's bedroom was 'just a mattress', uncle tells court

The court also heard from Ms Haigh's uncle, Michael Haigh, who described his niece as a "happy" and "smiley" girl.

Mr Haigh said he once visited the Geeveses' property alongside her father and saw that his niece's bedroom contained "just a mattress".

"It was probably four or five inches thick … [there was] no bedding, no cupboards," Mr Haigh said.

He said Ms Haigh once told him she was "scared" of Anne Geeves.

The trial continues.