The Fiji government needs to explain how hundreds of South Koreans have been allowed in to Fiji to work for firms connected to the controversial Grace Road Church, a union leader says.
Police are investigating the South Korean church's Fijian branch after its founder was arrested in Seoul for enslavement.
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Six people have reportedly been detained in Fiji and the church has taken legal action to stop deportations.
The Grace Road Group of companies runs dozens of businesses in Fiji employing at least 300 Koreans and 100 locals.
It also leased land at Navua for rice-growing.
Fiji Trades Union Congress National Secretary Felix Anthony said the case had exposed local immigration and labour authorities.
"We are very concerned .. and I think this has actually exposed our immigration department which issues work visas. It's exposed our Ministry of Labour for not keeping an eye to ensure that we don't condone forced labour in Fiji and I believe there needs to be a thorough investigation into this matter.
"Those responsible must be held to account," he said.
Mr Anthony said it was not the only company that was bringing in foreign workers.
"Work permits are issued willy-nilly despite the fact that we have workers in Fiji who could do [that] work and we have people unemployed in this country. So it's all about cheap labour."
He said the company was mainly based in rural areas and the union had no knowledge that there were foreign workers on its farm.
He said the ultimate responsibility was with the government.
Mr Anthony said the union had not been involved on behalf of the workers because of the language barrier and also because the workers involved were not willing to speak out.
The Grace Road Group runs restaurant chains, beauty salons, a construction firm and hardware store as well as a farm working in partnership with the Fiji National University.
"GR Group family, who has been working proudly as owners are very enraged by all the lies," the company said in a statement.
It said it carried out its operations "in the open".
"Those who wish to slander us have created unspeakable lies about passport confiscation, forced labour, incarceration and violence."