Bosses frequently take advantage of “ambiguity” in the award system to pay the nation’s most vulnerable workers rock-bottom rates, a workers’ advocate has warned.

The minimum award in each industry has various rungs, and it is the powerless – immigrants, women and the young – who are pushed to the bottom of the ladder, said a National Union of Workers (NUW) spokesman.

On Wednesday, the workplace watchdog announced that almost 200 clerical, home care and disability support workers employed by a health business in Darwin and Brisbane were “inadvertently” underpaid more than half a million dollars over four years.

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The company’s human resources manager made a “genuine error”, the Fair Work Ombudsman found. Once the employer was told of the mistake, it agreed to backpay the $502,000 in entitlements.

“It seems the HR manager was unaware how to calculate correct wages under the relevant Award and did not seek advice or clarification,” Ombudsman Natalie James said.

But bosses who employ powerless workers have both “leeway” and incentive to pay the barest minimum of the minimum wage, said a union official acquainted with the matter.

“It tends to be the employers who are paying the minimum rates of pay that can make these so-called honest mistakes,” said National Union of Workers (NUW) Victorian branch secretary Gary Maas.

Union-bargained agreements are lengthy and “tightly worded”, said Mr Maas, whereas the minimum awards are “no longer than a few pages” and full of “ambiguity” in the words used to describe different types of work, which can benefit employers, he said.

“If I was a boss, I’d be wanting to pay the lowest rate, and I think that that is probably what you’ve seen in this situation.

“Unfortunately, we see these types of so-called honest mistakes far too often, and a propensity of these mistakes to occur on non-union sites or sites which haven’t got a collective agreement in place which has those descriptors nutted out.”

Most non-compliance not deliberate, says watchdog

The Fair Work Ombudsman does not believe that a great many employers deliberately underpay their workers.

Last financial year, the Ombudsman resolved 25,650 complaints from employees and recovered $23 million for 15,483 underpaid workers nationally.

“Most” of these incidents were “actually inadvertent”, a Fair Work spokeswoman said.

“The large majority of employers want to do the right thing by their employees,” she said.

The real problem is one of complexity, said an employers’ group.

“In effect, the award system provides more than 1000 different minimum wages which are higher than the national minimum wage,” said Australian Industry Group CEO Innes Willox.

How workers can protect themselves

The “best defence” against underpayment or unfair treatment is knowledge of workplace rights, a Fair Work spokeswoman said.

Workers should also regularly check their payslips, as small errors can result in very large underpayments in they’re left unchecked.

“Keep a diary with a record of the dates of shifts worked, start and finish times and meal breaks to check that the pay slips and pay add up correctly,” she said.

An alternate view is that collective bargaining is the “best way around” the problem of barest minimum pay, a union official said.

“Sites which have collective agreements in place, which are negotiated by their union, will often have a very good classification system to decide the type of work that goes along with the rate of pay which is paid for that work,” said Mr Maas.