The union alleged the company's actions intended to undermine ongoing negotiations towards a new collective agreement.
The Employment Court declared that Affco's actions amounted to an unlawful lockout of its workers and found the company and meatworkers were in a continuous employment relationship which lasted throughout the off season.
The collective agreement created ongoing and enforceable rights and duties, including redundancy rights and an obligation on Affco to re-employ seasonal workers according to seniority.
The new terms offered by Affco sought to abolish the significance of seniority.
The Court of Appeal said the purpose of Affco's actions was to undermine the collective bargaining regime designed to redress the inherent inequality of power faced by workers in this situation.
The court concluded the unlawful lockout provisions must extend to protect former employees who had contractual rights to an offer of re-employment from an employer which was refusing to engage them unless they accepted new terms inconsistent with their existing rights.
The ruling concluded the employees' contracts made it clear their employment was terminated at the end of each season so meatworkers could not be in a continuous employment relationship which lasted throughout the off season, as the Employment Court ruled.
Union national secretary Graham Cooke said as a former slaughterman himself, he could feel what the Affco employees would be feeling.
"I know that many of them will be relieved that the Court of Appeal has backed up what the Employment Court decided a while back.
"This is a major advance in human rights for meat workers - and a significant step forward for collective bargaining rights generally. [Yesterday] Affco got a clear message that it needs to start negotiating in good faith."
Affco NZ executive director Dane Gerrard said in a statement the company was disappointed at the ruling and it was difficult for the company to accept anyone was locked out as the finding was based on a technical and inadvertent breach.
"At all times all workers were free to resume work but with the collective agreement having expired over 18 months earlier, it was on a revised set of terms. Those renewed terms provided higher earnings not less," Mr Gerrard's statement read.
Mr Gerard said the company welcomed the part of the Court of Appeal decision that ruled in Affco's favour, an issue about employment continuity for seasonal workers.
The court made no order to costs, saying its judgment was each party should bear their own costs.