The prosecution alleges it was Michael Curtis's brother Wiremu Curtis, who was the partner of Nia's mother at the time.
Mr Simiona initially watched as a small child was put carefully onto the bars of the line and whirled around gently by Michael Curtis while the other two adults looked on.
A short time later, the child was lifted off "with some affection, if you could call it that".
Meanwhile, the witness heard a little girl "moaning she wanted to get up on the line too".
Kemp and one of the men forcefully told her to shut up but she kept on complaining she wanted a turn.
The witness said he then saw Michael Curtis grab the young girl (allegedly Nia) under the armpit and "chuck her on".
She clung to the centre pole as Curtis spun the line at an increasing pace.
At first she was laughing but then began screaming to get off.
"The line just got faster and faster."
Michael Curtis kept on swinging it as Kemp and the other male stood watching, Mr Simiona said.
"She flew off the line backwards. I heard them laughing and saying `see what happens when you want to get on the line and be a humbug'."
He said he was walking over to the fence when he saw Michael Curtis, who was holding the toddler by the leg, fling her back onto the clothesline.
She landed on the wires between the two lateral bars and "just tried to grab onto anything she could".
Before long, the girl fell through the wires back onto the ground while the adults stood and laughed.
"She was just screaming - she was crying, crying."
That was when he confronted them, the witness said. They said nothing as he walked back inside still swearing.
Michelle Bartlett, a former neighbour of the Curtis brothers, their partners and children, said she and her family moved next door at the end of June last year.
The mother-of-three said she would hear a baby screaming and crying at night "all the time".
Usually starting about 8pm, she would hear it when she went out to her verandah for her hourly cigarette.
Ms Bartlett said she mostly went to bed about 1am or 2am when the crying stopped.
Asked by Crown prosecutor Amanda Gordon how she could determine which house the screaming came from, the witness said: "I just knew".
Each time it was the same type of cry.
"I have heard a lot of children crying before. It was a hurting cry, not a hungry or I'm tired (cry). It was a scream."
Ms Bartlett said she stayed up until it stopped because it was annoying.
"I didn't know what to do. I could have rung somebody or done something but I never."
She had not seen any adults at the house but would hear "quite loud, quite basey music" late in the evenings. It drowned out any other noise from the property.
Michael Curtis, 22, and Wiremu Curtis, 18, have pleaded not guilty to murdering Nia.
Nia's mother Lisa Kuka, 35, Michael Pearson, 20, and Kemp, 18, have pleaded not guilty to manslaughter.
The trial continues tomorrow
- NZPA
