The Ministry of Business Innovation and Employment says tackling exploitation of migrant workers in the construction industry is challenging because there are so many different trades involved.

A popular Christchurch Indian restaurant chain's owner has had his company assets frozen as he faces charges of worker exploitation.

Amar Deep Singh, who built a mini empire of Coriander's Ethnic Indian Restaurants over the past 12 years, denies any wrongdoing.

His three restaurants – in central Christchurch, Upper Riccarton and Rolleston – continue to trade, but their future is uncertain.

ALDEN WILLIAMS/STUFF The owner of Coriander's Ethnic Indian Restaurants will face charges of worker exploitation in May. In December his assets were frozen but the businesses are still trading.

Extensive investigations over several months by the Labour Inspectorate concluded employees had been "systematically underpaid and abused", court documents show.

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The case against Singh is scheduled for May 19 and 20 at the Employment Court in Christchurch.

Carys Monteath/Stuff Amar Deep Singh pictured at the Rolleston Restaurant in 2007.

The Inspectorate is seeking recovery of pay, a declaration of breach, penalties and a banning order.

Singh was not present at a December 13 hearing when the court first froze his assets to ensure funds were available for payment of arrears and "significant penalties", if needed.

The order, which was made permanent on December 19, does not affect funds for payment of staff and normal trading costs.

ALDEN WILLIAMS/STUFF On Friday, all three restaurants were listed for sale with NAI Harcourts for between $250,000 and $650,000.

On Friday, all three restaurants were listed for sale with NAI Harcourts for between $250,000 and $650,000.

Speaking from India, where he has been living since January 2019, Singh denied he had underpaid staff.

He said some staff had made up stories about their treatment because they held a grudge against him.

"The people that worked for me were always paid well."

The businessman said he was very passionate about his restaurants and loved Christchurch.

He went to India to spend time with a sick uncle and was intending to appear at the May hearing.

After moving to New Zealand in 1999, Singh opened his first restaurant in Rolleston in 2007 after working at another restaurant, Little India.

One staff member, who did not want to be identified, said she had never been underpaid but was now concerned about her future in New Zealand.

Most of the restaurant employees were on temporary work or study visas – many tied to Coriander's.

The worker had spent $40,000 on hospitality studies and $8000 towards a residency application.

"I spoke to Immigration [NZ] and they said if the company is in a situation of selling or liquidating, then the visa will end right there. I would have no rights to live in the country, so it's very difficult for us."

Indian Workers Association and Union Network of Migrants Mandeep Bela said speaking out about exploitation was extremely difficult for migrant workers, especially when their temporary work visa was tied to one employer.

"It is a daunting task for many migrants, it's absolutely really difficult."

Migrants faced the threat of losing their job, being forced to return to their country, and letting down dependent family members in their home country.

"We applaud those migrants who actually make the decision to counter exploitation," Bela said.

He hoped affected Coriander's employees were given the opportunity to stay in New Zealand while the case was being heard.