Ms Armstrong said she could hear yelling on the other end of the line at the start, and Kuka replied, "Hang on a minute, hang on a minute, it wasn't me."
After a pause, Kuka then allegedly said, "I saw it happen. I saw it. I was there."
Ms Armstrong said there was another pause before Kuka said: "He put her head between his knees and he hit her head on the floor three times.
"He threw her across the room and she hit the wall because he said she was ugly and won't listen."
Ms Armstrong was deeply upset by what she heard and said she "turned around and went into a back room because I thought I was going to be sick".
Under cross-examination, the ward clerk said there had been no mistaking what Kuka said because she was speaking slowly, calmly and quite loudly.
Later that day a nurse who was looking after Nia told Ms Armstrong no one was sure how Nia's injuries had occurred because no one in Nia's family was talking.
"My reaction was 'Mum saw it' ... I was really upset about it, moreupset when I realised no one knew about it."
It was then that she wrote her notes about the conversation and told police.
Kuka, 35, is charged with Nia's manslaughter and has denied failing to get her daughter medical treatment and failing to protect her from violence.
It is alleged that Kuka's boyfriend, Wiremu Curtis, aged 19, and his brother Michael, 22, murdered the toddler.
Michael's girlfriend, Oriwa Kemp, 18, and Nia's cousin Michael Pearson, 20, are also charged with the 3-year-old's manslaughter.
It was revealed in evidence yesterday that Kuka had been in a relationship with Wiremu's and Michael's older brother, Shane (also known asJules), before she began seeing Wiremu.