The rail workers' union claims more than two dozen Chinese engineers could still be being exploited in New Zealand.
The Rail and Maritime Transport Union (RMTU) wants the Employment Court to decide if foreign workers are subject to our employment law.
It comes after claims 20 Chinese workers were being paid $4 an hour by KiwiRail last year, and living in cramped conditions.
Workplace Relations Minister Michael Woodhouse said the best advice was that it was "probably legal", but a definitive answer would have to come from the courts.
RMTU lawyer Geoff Davenport told an Employment Relations Authority hearing in Wellington today that 27 Chinese engineers were working in New Zealand right now, and they could also be in the same situation.
He said there was nothing to stop a New Zealand employer bringing in foreign staff, paying them way less than the minimum wage and not giving them annual leave.
Mr Davenport said that would destabilise New Zealand workers and the New Zealand workforce.
KiwiRail lawyer Peter Chemis argued the question in this instance is based on very specific facts, not the fundamentals of the law.
He said the Ministry of Business, Innovation and Employment had twice looked at the case and decided there's "nothing to see here".
Mr Chemis said KiwiRail was pushing for the ministry to look at the case a third time.
Authority member Michele Ryan said she hoped to make a decision on whether the case would be dealt with by the Employment Court within six weeks.