The Service and Food Workers' Union says it fears thousands of employees are being paid less than the minimum wage but are too scared to come forward and complain.
The Business, Innovation and Employment Ministry says it received 593 complaints last year about employees being paid less than the minimum wage.
Of those, it says, nearly half were resolved in favour of the employee. Most of those not upheld were voided when the employer voluntarily paid up or went out of business.
The union says that number is just the tip of the iceberg.
Union secretary John Ryall says many migrant workers are forced to work long hours for little pay but fear that if they speak up they'll lose their jobs or work visas.
He says the problem is getting worse as the Government puts pressure on workers' rights.
Mr Ryall says it's vital the country develops a culture where the exploitation of workers is unacceptable.