While workers were instructed to wear the hat, to improve on a 10 per cent head injury rate throughout Silver Fern plants, if workers felt they were overheating they could ask their supervisor if they could remove it.
The plastic Protector Bumpmaster hat was worn over another hat workers were required to wear for hygiene reasons. The company acknowledged heat was an issue which was why it could be removed if heat stress looked to be a greater risk than head injury.
In other parts of the plant, where falling objects were an issue, sturdier hard hats were compulsory at all times.
Mr Kupa was stood down by assistant plant manager Frank Elliot on March 25 last year after a managerial tour of the plant.
After an investigation he was later dismissed, having received no written warnings.
Mr Elliot told the court there were heat and ventilation issues where Mr Kupa worked and Mr Kupa had previously expressed "ardent views" against the introduction of the hats as mandatory personal protection equipment.
On March 25 he said he observed Mr Kupa hatless early on his morning shift when there was little chance of him overheating.
He asked Mr Kupa's supervisor to address the matter, who reported back that Mr Kupa said he was too hot to wear the hat.
Co-workers also took off their hats but complied after Mr Elliot repeatedly told them it was not hot and they would be refusing "a reasonable supervisory request" if they did not wear them.
Mr Elliot said Mr Kupa interrupted the conversation with one colleague, accusing Mr Elliot of "bullying and intimidating".
Mr Elliot said he asked Mr Kupa to put on his hat "but he refused, saying it's too hot".
After several investigation meetings Mr Kupa was dismissed, despite having a clean record and receiving no written warnings.
The case continues today for its third and final day.
Mr Kupa played 60 first class rugby games between 1980 and 1992.
