It follows the introduction in October last year of a law that took away the legal right to a tea break. The Employment Relations Amendment Bill also weakened collective bargaining.
Urges to boycott the Australian fashion chain's New Zealand stores came from many Twitter users.
Puawai Cairns wrote: "Damn you, Cotton On. I'm going to have to boycott you."
Smithers @TedTedwin wrote: "Will now boycott cotton on for taking advantage of legislation that hurts workers rights" and Luqman Hayes wrote: "Cotton On: Keep your workers' tea breaks or we we will boycott."
On the attack
Labour had gone on the attack earlier today, saying responsibility rested with the Government, but Workplace Relations Minister Michael Woodhouse said if the workers' union did not like the conditions they could simply say "no".
Mr Woodhouse had said he would not comment on specific negotiations, but said recent changes had simply returned the law to what it had been before the last Labour government.
"There is a very straightforward response to a vocal and confident union that doesn't like the idea of building flexibility into the agreement - and that is, just say 'No'."
Labour leader Andrew Little said those comments showed Mr Woodhouse didn't understand how such negotiations took place. "Cotton On is doing what big corporate employers tend to do - look at the law and see what they can get away with.
"If you write a law that allows employers to take away rest breaks and meal breaks, don't be surprised when employers then apply the law. The responsibility for this lies entirely at the Government's feet."
Business New Zealand manager of employment relations policy Paul Mckay told Radio New Zealand the law did not prevent workers from taking lunch breaks at a time negotiated by the employer and worker. "It's not a case of no breaks, it's a case of working out when those breaks can be taken."
He said workers who finish their shift without taking breaks were then entitled to compensation, he said.
PM 'misleading'
First Union secretary Robert Reid said the retail workers union was negotiating a collective agreement for staff at Cotton On's distribution centre in Auckland.
"The Prime Minister reassured New Zealanders that 'post the passing of this law, will you all of a sudden find thousands of workers who are denied having a tea break? The answer is absolutely not'. We now know the Prime Minister's assurance was misleading," Mr Reid said.
"Negotiations began in July last year and both parties agreed to paid tea and meal breaks. But after the government's law changes came into force Cotton On has submitted a late claim to remove tea and meal breaks.
"Cotton On is trying to take advantage of a law that was always meant to strip workers of their rights."
Mr Reid said breaks are crucial on industrial sites because they keep people safe and worker fatigue was a risk at the distribution centre.
"Yet this has not stopped Cotton On trying to exploit the new law to its own advantage. This is what the government always intended, an economy where our competitive advantage is poor working conditions."
