The claims included receiving an unjustified warning over a damaged vehicle when she wasn't present when the damage occurred, working overtime, and having her duties and place of work altered.
Ms Harrington also said she was sexually harassed by a senior employee, which involved "inappropriate touching".
When she brought the matter up with management, she was told an investigation would take place after Christmas "combined with a comment the complaint could result in counter allegations and disciplinary action against Ms Harrington".
"Such a threat is totally improper," said ERA member Michael Loftus.
Ms Harrington also said she was discriminated against because of her gender during her work for the company.
During one incident on 15 December 2008, she was asked to perform duties beyond her capabilities, such as moving a load of about 800kg
She tried briefly before refusing, and then received threats from colleagues they would complain to management about her "inadequate performance", said the finding.
When trying to move the load, she felt a stabbing pain.
Two days later the pain had become acute but when she sought assistance from her manager "this led to derision and laughter from other employees".
Ms Harrington said she was denied access to first aid and was told if she called an ambulance it would result in disciplinary action.
Her father took her to the doctor who said she should urgently go to the hospital in Queenstown where she stayed for two days.
The final incident happened on December 29 when a colleague made a comment she found inappropriate and hurtful about her recent illness.
Immediately afterwards, she resigned.
Company conduct forced Ms Harrington's resignation, said Mr Loftus who referred to failures to address the sexual harassment complaint and failure to assist with access to medical care "in the face of what was an obvious and serious condition".
Ms Harrington's lawyer Angela Boniface said her client was happy with the decision.
"All we really want to say is that we're pleased at the outcome, signalling the end of a long battle."
Justin Marshall, who was managing director of Thunderbird One when Ms Harrington worked for the company, described her claims as "outrageous" and said he was surprised by the decision.
"The finding is quite bizarre," he said.
He declined to comment further.