More than 500 retail workers have already complained to the union about unpaid work in the wake of the Smiths City decision.
On Friday the Employment Court ordered the electronic retailer to pay staff for their attendance at "voluntary" sales meetings over the past six years.
Tali Williams, First Union's retail, finance and commerce secretary, said it had had 500 or so responses since it had released a survey on Monday asking workers about whether they are asked to come in for meetings before or after work which are unpaid.
"The nature of a lot of the complaints is either meetings at the start of the day or at the end of the day staying behind to clean."
Williams said it was now trying to figure out which retailers were the worst culprits. It had received complaints about Cotton On, Briscoes and Harvey Norman.
The union would be following up the complaints with the individuals and then the retailers once it knew the specifics of the case.
Williams said she imagined a lot of retailers would already be responding to the Smiths City decision.
"But that doesn't mean they get off what has happened for the last six or seven years."
Williams said it had heard anecdotal evidence over the years about staff being asked to come in early for meetings and not being paid for it but retailers had previously argued that it was just part of doing the job.
But the Smiths City ruling had drawn a line in the sand, she said.
For at least 15 years, Smiths City Group, which has about 400 store-based employees, had been holding 15-minute sales meetings before opening its 34 stores around the country each day.
The meetings, which the company claimed were optional to attend, were unpaid.
However, an Employment Court decision showed there was very much an expectation to attend, with staff being questioned if they were late or absent.
"A telling observation was made by the manager who gave evidence about this expectation,'' the decision read.
"They regarded those who did not regularly attend the meetings as tending to be poorer performers and that attitude was undoubtedly conveyed to sales staff.
"In combination, the company's expectation to attend and the managers' attitude about those who did not, created pressure that brought results.''
The Ministry of Business, Innovation and Employment released a decision by the Employment Court saying that if the activity - in this case, the meetings - are integral to a worker's role and that there is an expectation to attend, then that is considered work and employees should be paid for it.
In a statement released by Smiths City, the company said it now accepts the Employment Court's decision that the meetings constituted work as defined by the Minimum Wage Act.
Roy Campbell, Smiths City chief executive, said: "We have now moved the sales meetings into employees' normal working hours. We are complying with the Employment Court order that we conduct an audit to identify where wages have been paid below the statutory minimum. The audit is covering all current and previous employees for the last six years. We will calculate the arrears of pay below the minimum wage and reimburse any affected employees accordingly."
Labour Inspectorate regional manager Loua Ward said employers should not pass the cost of doing business on to the people on the floor.
"Employees must be paid for all the work they do and this includes handover times, briefing and in some situations, the travel time to and from a work site.
"Too often we encounter employers attempting to avoid paying their employees by dressing up activities outside of business hours as something that is for the benefit of the employee or something that's not work,'' Ward said.
"However, we will look beyond that at the real nature of the activity.''