In an opinion piece written for Newshub, the woman - who also gave evidence during the trial - wrote that her instincts told her something was not right when she met the murderer for a beer.
"I want to share my experience so other women can remove themselves from situations they feel unsafe in."
When the two met at Ponsonby's Revelry bar for a beer on Sunday, December 2 last year, he told the woman that he'd been shopping for a suitcase or a bag with wheels and was frustrated that he couldn't find one big enough to fit all of his sports equipment.
At Millane's murder trial, the jury was told that the date was arranged via Tinder and confirmed earlier in the day while Millane lay dead in his apartment.
The Crown said that when the killer met his date at the Ponsonby bar, he had already put Millane's remains in a suitcase, which he then moved to the boot of a rental car he had hired.
The previous evening, British backpacker Grace Millane was strangled to death on the eve of her 22nd birthday by her Tinder date in his downtown Auckland apartment.
The woman wrote for Newshub that he looked like a "solid guy who played sport well".
"It makes me quite sick now to think of what that suitcase was actually used for."
She found the killer to be aloof, writing that he wasn't there to make a connection or to find out more about her.
She said that later on in their date, he "became very energised" when talking about poisonous snakes in Australia, telling her about how brown snakes killed a man in his house, and another on a farm.
"It was as if he was quite taken by morbidity, by death and by violence," she wrote.
He told her that his best mate was also a Crown prosecutor who would shortly be moving from Auckland to Sydney.
"He then told me a random fact - that his police friends were having a tough time at the moment as lots of bodies were being buried in the Waitakeres and, as police dogs can't smell deeper than four feet, it's hard to find them."
In an effort to insert herself into the conversation, she said she mentioned court trials she had sat through when she was a journalist, including one when she saw a young man being put away for murder.
Hearing this, he told her, "it's funny how guys can make one wrong move and go to jail for the rest of their lives," she wrote.
He said, "I heard of this guy whose partner asked to have rough sex with him involving some sort of strangulation and suffocation but it went wrong and the guy couldn't revive her and she died. He got done for manslaughter but it was really tough for him to see this woman he loved dying."
He drank his beer quickly and was on his second drink when she went to get water for the table, she wrote.
She worried that he would take off with her bag and she left the table, and wrote that she was careful never to leave her drink with him.
"For some reason, I felt like he was the type of guy who would spike it. I felt bad for thinking that though."
After around an hour and a-half, the two left Revelry.
She said that he told her his car was down a specific street, right where she had parked.
But feeling unsafe around him, she lied and said that her car was parked down another street and walked away.
She wishes that she had known something and had been able to do something for Grace, she wrote.
"How on Earth could this man be having a drink with me while her body was back in his apartment?"
"I will never ever be able to come to terms with that.
"I'm left with a sense of hopelessness that there was nothing I could do to help Grace."
She does not think she was in danger and said that she has very strong instincts about people, which she trusts, she wrote.
She wanted to tell everyone that they never have to put themselves in situation that they feel uncomfortable or unsafe in, she wrote.
After a nearly-three-week trial, the jury unanimously returned with a guilty verdict for the man accused of Millane's murder on November 22.
During his trial the killer claimed Millane instigated the choking and effectively introduced him to it.
The jury heard she had an interest in BDSM but, ultimately, believed the Crown case that Millane had been murdered.
