However, general manager for Allied Concrete Grant Driver "strongly" denies exploiting any worker employed by the company.
"Allied pay our workers fairly with industry leading wage rates, and no differential based on any race, gender, religious beliefs or any basis whatsoever," he said.
First Union and Migrante New Zealand spokesman Mikee Santos said initially the offers look good.
"But what we see happen is that those hours aren't enough to complete the job so they end up working for longer for the same money and this brings down their hourly rate.
"They are paid for set hours, but frequently work outside these hours."
Santos said the tactic used was too common and left migrant workers with few choices.
"Filipino migrant workers face ramifications from the local agency that sent them to New Zealand (who they often owe money to), and from their New Zealand employer should they be unhappy with anything that employer asks because their visa is locked into that employer.
"They have to secure a new visa if they wish to change employers. It's a modern form of exploitation that urgently needs attention."
The employer was using the fact the workers were unaware of their rights, and that their visas were tied to that single employer, to exploit them, Santos said.
"This is why we urge migrants to join a union so they are aware of their employment rights as part of their assimilation to New Zealand. I cannot stress this enough."
Driver told the Herald most staff work overtime on an agreed basis and are offered and reimbursed for any overtime worked.
He also said Allied Concrete presented the union with a 3 per cent increase on top of wage rates and all claims were agreed to except one.
"To now find the union is striking after they have been offered the rate increase they requested has mystified and disappointed Allied Concrete.
"We are a good employer to our great people and we have had great employee/employer relations for more than 20 years.
"As the company has met all but one of the claims, Allied Concrete are unsure of the reason for the strike or what the Union seeks as they have not communicated with the Company since July," he said.
A recent report found Filipino migrant construction workers in Auckland and Christchurch were paid less than their Kiwi counterparts and living in "unhealthy" conditions.
The research, commissioned by union E Tū, recorded the experiences of mostly Filipino, construction workers in Christchurch and Auckland in 2017 and 2018.
Most experienced pay discrimination, with some earning as low as $19 an hour, compared with $35 paid to New Zealand workers.
