Australian jobs advertised in foreign languages overwhelmingly offer illegally low rates of pay, a union has discovered.

Unions NSW published a report on Monday alleging that 78 per cent of the online ads it translated from Chinese, Korean and Spanish were underpaid, at an average of $5 below the legal minimum.

It described the problem as “endemic” and “gross exploitation”.

“The results are a snapshot in time of underpayments and provide strong evidence of endemic underpayments of workers from culturally and linguistically diverse backgrounds,” Unions NSW wrote.

The report found that the cleaning, hospitality and retail sectors were the worst affected, with the average underpaid job paying just $14 an hour.

The peak body conducted two audits, one in March 2016 and one in April 2017, of 200 randomly selected online job advertisements.

Over that time, the problem appeared to worsen. In 2016, 74 per cent of jobs were advertised below the relevant award rate, which increased to 83 per cent in 2017. Combined, 78 per cent of the advertised jobs were underpaid.

All up, the union translated 87 ads on Chinese website Sydney Today (46 in 2016 and 41 in 2017); 72 from Korean website Hojunara (38 in 2016 and 34 in 2017), and 41 from a number of Spanish-language Facebook pages (21 in 2016 and 20 in 2017).

Workers who successfully applied for the underpaid positions probably lost other entitlements such as superannuation as well, the union warned.

“Underpayment of temporary migrant workers tends to coincide with the undermining of other labour standards and workplace conditions.”

The union inferred from the way the job ads were worded, that the victims of underpayment were likely to be young students with poor English working on temporary visas.

“Migrants and visitors who do not speak English often rely on other members of their community for information. This relationship of inter-community dependence allows nefarious employers to take advantage of recent migrants and workers from culturally and linguistically diverse backgrounds.”

Unions NSW used the report to call for an amendment to the Fair Work Act to once again allow unions to inspect the full pay records of any business suspected of exploitation.

“Unions are now only able to check the pay records of union members. The barriers culturally and linguistically diverse workers face in making complaints often also hinder union membership where workers are scared to ‘rock the boat’.”

It also called for harsher penalties for repeat offenders.

“The current approach to stamping out underpayment of culturally and linguistically diverse workers under the Fair Work Act is not working. A shift is needed to remove the onus on individual workers to make complaints towards providing opportunities for unions to empower workers to collectively challenge underpayments.”

In the absence of law change, Unions NSW has started naming and shaming allegedly underpaying businesses on its Wage Thieves website.