Wage theft is common for retail staff and other industries, a workers' union says.

It comes after revelations that thousands of Woolworths employees may have been underpaid over several years.

A review in early 2022 found some had worked extra hours for free, Woolworths said.

"We've made contact with around 5000 former or current salaried store team members who worked for us between 2015-2022 and who had roles that could potentially have been impacted by this.

"As we're just starting this process, there isn't enough information available for us to determine exactly which team members or stores may have been impacted and to what extent."

In some instances, staff had not been paid correctly for extra hours, or worked on days recorded as annual leave.

First Union chief executive Bill Bradford said senior management deserved credit for their commitment to remediating these workers.

But he said wage theft was rife.

"This is a problem when employers try and get workers to do extra work that they are not paid for, and that ranges through from being asked to report 15 minutes early and they're not getting paid for it or being asked to complete a job for another half an hour when they've clocked out."

Bradford said migrant workers, whose visas were tied to one employer, were particularly vulnerable to exploitation.

They were not always good at sticking up for their rights and did not want to "make a fuss" without having a permanent residency.

This story was originally published by RNZ