Labour Party MP Kieran McAnulty says he is "emotionally tied" to fixing worker exploitation.

Employment law could be changed to improve the treatment of temporary workers recruited via agencies.

The Employment Relations (Triangular Employment) Amendment Bill, suggesting an amendment of the Employment Act, passed its first reading in Parliament on Wednesday night. The Labour, Green and NZ First parties all supported the Bill. National did not.

If enacted, it would give more rights to thousands of workers, often migrants, hired through supply agencies.

KEVIN STENT/STUFF National workplace relations spokesman Michael Woodhouse says the rest of Labour's proposed employment law changes are "union-friendly".

Labour MP Kieran McAnulty​, who resurfaced the near decade-old bill last month, said it was still needed. The exploitation of subcontracted workers had become "appalling", he said.

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Labour-hire workers were being fired without warning, often for "unacceptable" reasons like falling pregnant, he said.

Companies were using "rogue" labour-hire companies to pick workers based on their ethnicity, sex and age, he said.

"That is pretty disgraceful and it's unacceptable in this country," McAnulty​ said last month.

The bill would give labour-hire workers power to bring a personal grievance against the company they are subcontracted to work for.

First Union spokeswoman Lisa Fox said the entire labour-hire industry had been operating outside the law for too long.

Almost all of the labour-hire workers that came to the union had clauses in their employment contracts that let them be dismissed without notice or reason, she said.

Others had worked for companies for nearly a decade, but had never been offered permanent work contracts, she said. Those workers had not received any annual leave or paid sick leave.

If enacted, the bill would ensure thousands of workers hired through labour-hire firms would see fair working terms, she said.

McAnulty​ said he was disappointed National did not support the Bill, but "absolutely delighted" it passed to the education and workforce committee regardless.