Authority member Andrew Dallas said there was "compelling" evidence about the hardship she suffered and ordered the law firm to pay her $25,000 compensation.
Problems first arose around a closed client file.
The work, completed by a departed solicitor, had not been invoiced and the electronic file had gone missing.
The client, the ERA heard, had been known to Morrison and the solicitor told company principal Tina Williams it was a "Jo Morrison special".
"The implication for the use of this phrase is that there was some sort of wrongdoing on the part of Ms Morrison," Dallas said.
The night after the document disappearance was raised, Morrison worked with an IT expert and they recovered it from her computer.
The next day, after she explained to Williams and the other company principal, Joanne Westgate, she was suspended, despite her protestations.
A letter a few days later alleged Morrison had gone into work after-hours and used the computer system without authority; she had used a private email for work business, and accessed work emails on her personal phone.
She was warned, it "could amount to serious misconduct, which if substantiated could result in her dismissal".
On August 3, 2017, there was a disciplinary meeting during which Morrison provided detailed responses to the allegations.
A week later, she received a letter from Westgate & Williams saying the claims had been substantiated and the preliminary view was that she should be dismissed.
On August 15, Morrison was fired.
Her employers had "lost trust" in her, they said.
But when the case was heard before the ERA, Williams said she had not reached a conclusion that Morrison deleted the file and could not confirm who had.
Dallas was scathing of the inquiry taken on by Williams and Westgate.
"Despite believing Morrison to be a 'trusted employee' and her alleged conduct 'seemed so out of character', I accept, as a general proposition, Ms Morrison's submission that [the company] closed its mind to the genuineness of her explanations," he said.
The firm failed to properly investigate the allegations, he said, by not seeking to interview several relevant witnesses.
And Williams should not have been involved as a decision-maker because of her role as a witness, the ruling found.
Westgate & Williams was ordered to pay Morrison $25,000 compensation, $4660 holiday pay and $1500 for the lack of employment agreement.
It was also fined $500.