In a personal grievance case raised the following month, Mr Casson said he felt he had to resign due to "bullying and threatening behaviour" by members of the Hamilton police management who had "closed ranks" against him.
He alleged an inspector had threatened to transfer him, and threatened staff to distance themselves from Ms Casson.
Mr Casson also alleged another inspector had instructed a senior sergeant to "sort you [Mr Casson] out".
Police argued before the ERA that Mr Casson did not raise his personal grievance within the 90 days he was required to by law.
But Mr Casson said he did not raise matters at the time because he felt intimidated and was afraid of retaliation.
In a preliminary determination, released today, the ERA said each of the separate allegations were not raised as personal grievances at the time.
But the ERA allowed Mr Casson to raise a narrow claim of unjustified disadvantage for allegedly being treated unfairly, targeted for no reason, and being made to feel marginalised and bullied.
The authority said he could refer to the various events alleged in his personal grievance claim, even though they were not grievances on their own, as background evidence at a substantive hearing, which was yet to be held.
The ERA said that hearing would cover much of the same ground as the preliminarily hearing, and urged the parties to seriously consider going back to mediation.
Meanwhile, the ERA has ordered Tasman district police to re-hire its business services manager after finding his redundancy was invalid.
Derek Coffey had worked for police for 23 years before losing his job in February as part of a nationwide police restructure.
The ERA found his redundancy was invalid and he was unjustifiably disadvantaged by police's failure to redeploy him to an available suitable position.
It ordered police to reinstate Mr Coffey and to pay him lost wages and $15,000 in compensation.
