Mr Niuapu stepped in and told him not to speak to a woman in such a manner and "do your job right, you 60-year-old man". Mr Buchanan then punched him in the face.
A series of disciplinary meetings were held that day with the employees in question and senior staff at the plant.
Afterwards, operations manager Michael Manson deemed Mr Buchanan's actions warranted an immediate dismissal.
Attempts by a union secretary to get the decision revoked in favour of a final warning and suspension for the remainder of the 2013/2014 season failed.
Mr Buchanan, who has since moved to Christchurch in search of better opportunities, claimed his former employer had failed to properly investigate the incident.
He said it had failed to consider all mitigating circumstances, including stress related to his mother being seriously unwell and care arrangements for his daughter.
He also felt the company had failed to give him a proper opportunity to respond and address what had occurred.
However, as detailed in the ERA decision, senior management felt his actions were unprovoked and, although his mother was unwell, this did not warrant his lashing out and thus upheld its decision to dismiss him.
The ERA ruled Alliance Group had done "more than was necessary" to investigate the circumstances of the misconduct and had given him ample opportunity to respond.
Costs were reserved for the parties to deal with between themselves.