But police denied this and argued that even if Opai was disadvantaged, it was justified because a restructure of the public counter resulted in the disestablishment of her position in a procedurally fair manner.
It also said her performance appraisal did not disadvantage her and that Opai's supervisor was justified in discussing performance concerns with her in an informal manner.
The ERA found Opai succeeded with two of her three unjustified disadvantage grievance claims. She failed in her unjustified disadvantage claim regarding the disestablishment of her watchhouse officer role as a result of the 2014 restructure.
The authority found that the disestablishment of her position had been done in a "procedurally fair manner" and the police had genuine reasons based on reasonable grounds for doing so.
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But it found Opai was unjustifiably disadvantaged in her meeting with Senior Sergeant Mullin on March 3, 2015.
"The performance concerns he raised, the way he did that and his advice to her that the next step was a formal performance improvement plan, was not what a fair and reasonable employer could have done in all the circumstances," the authority said.
"Police should have taken adequate steps to investigate the factual basis for Senior Sergeant Mullin's performance concerns, in order to ascertain whether they were fairly and reasonably at the point where it was appropriate for him to have advised Ms Opai that the 'next step' would be a formal process. That did not occur, which was unfair to Ms Opai."
She also succeeded in her claim about the police handling of her 2014/2015 performance appraisal.
"She was left without a finalised performance appraisal for the 2012/2013, 2013/2014 and 2014/2015 performance years. Her specific concerns were not adequately investigated or addressed."
For humiliation, loss of dignity and injury to her feelings, ERA found Opai was entitled to a compensation of $20,000.
Her health was significantly affected after she formed a subjective belief the police were wanting to remove her from her employment.
The unjustified disadvantages she suffered played into her worst fears about the police improperly targeting her and increased her sense of grievance towards police, the judgment said.
Opai said she was also psychologically affected after a March 3 meeting with Mullin about her performance, and the way she had been treated by Mullin created esteem issues in her and destroyed her passion for the job.
Earlier in 2012 and 2013, Opai had raised various concerns and made formal complaints against five of her colleagues.
Defamation proceedings was launched by her against Senior Sergeant Laurie Culpan and the Attorney General regarding comments Culpan made about her in a performance appraisal, a briefing paper, a complaint made about her and in diary notes in 2013.
However, she did not succeed in her claims - which she initially sought $280,000 in aggravated damages from Culpan for.
Then in 2013, she was subject of a complaint, Opai's claims against the police over this did not succeed. She also sought the same amount and further damages from the Attorney General for "vicarious" defamation.
The High Court dismissed the case against Culpan and awarded him costs of more than $25,000.
The ERA said: "These matters are set out to demonstrate there had been a lengthy history of issues involving these parties before the current disadvantage grievances arose."
