Patea, enraged at his ex, ran her off the road minutes after she dropped the little girl off at day care, and then beat her to death using a 7.8kg metal fire hydrant cover.
Leesa Kennedy saw it all - and spoke today in the Brisbane Supreme Court at Patea's sentencing.
Kennedy said she came out of her Gold Coast home after hearing the car crash.
She saw Patea at Brown's driver's window and assumed he was trying to help her.
Then, she realised he was killing her.
"What the f**k are you doing?" she screamed at him.
"She's got my kid," Patea replied, then carried on with the fatal beating.
He hit Brown with the metal cover 29 times before leaving it lying on her face and fleeing the scene in a stolen car.
Brown died the next day in hospital. Her injuries were described as "not survivable" and included facial and skull fractures and a laceration to her brain.
At Patea's sentencing today Kennedy read a victim impact statement.
The ongoing effect of witnessing Brown's death was clear to see and hear as Kennedy read her statement, facing Patea as he sat in the dock emotionless.
Kennedy explained that she had been witness to death and accidents before - but nothing prepared her for what she saw in September 2015.
"What I saw you do to Tara has horrified me to my core," she said.
"Initially thinking I was running to the scene of a car accident ... [it] was no accident. It was a domestic violence situation that resulted in you beating to death a young woman, as she lay trapped in her car, unable to help herself."
Kennedy said she struggled to process what she was witnessing.
"Even though I tried my best to stop you, I felt helpless and hopeless," she said to Patea.
For the six months after the murder Kennedy said every time she shut her eyes she saw Brown.
"With my eyes closed, she is screaming at me for help, just like she was ... when I arrived at her car and you were already there.
"My eyes would well up with tears because of the guilty that I will forever carry that I failed her in not recognising and understanding her fear was of you.
"She was screaming at me to help her from you - not being trapped in the car after the accident."
Kennedy tried to stop Patea but he pulled her away, throwing her into the smashed windscreen so he could carry on beating Brown.
She was pulling glass out of her skin for months from that windscreen, but said her mental injuries were "much worse".
"Some days it's like a repeat button gets stuck in my head as I remember every hit and swing of your arm," she said.
"The first thing I think of, every single morning when I wake up, is Tara and your daughter."
Kennedy said witnessing the murder put pressure on her relationship with her husband and children and resulted in her having "vicious nightmares about horrific murders" of her loved ones.
"I have not slept a full night since, and when I wake up, my thoughts turn to you and my actions that day.
"Did I do my best? Did I try hard enough? Why? How?
"Initially I felt like I had failed Tara and as I did not stop you, failed as a decent human being.
"For a time I felt unworthy of life, that I had allowed someone to die on my watch by not doing my best."
Kennedy said she often thought of Brown's little girl and was deeply affected by what the child would miss out on in life by not having a mother.
"Your choice to brutally kill Tara has resulted in your daughter not only losing her mother but also losing her father.
"I can't understand why you did that to your daughter - I don't think I ever will."
Another witness who lived near the murder scene wrote a victim impact statement for the court.
He did not want to read it to Patea, but Justice Debra Mullins allowed the Herald access to it.
The man's name cannot be published.
"I was very upset that even though I did what I could, Tara still died," he said.
"I won't say I feel guilt, it is maybe regret."
He said the attack happened so quickly he didn't realise what was happening at first.
When he did realise Brown was being attacked, it was too late.
"I was in shock, I was very angry," the man said.
"I have tried not to think about this incident every day. I have tried to move on with my life.
"There are days I do think about it."
A cross has been erected near where Brown died and the man visits it often.
"I will say hello to Tara when I walk past. I do this to honour her memory and I water the plant that has been left there for her."
