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.