The Maori Party says it cannot support the Government's controversial employment bill as it will hurt workers.
Parliament is scheduled tonight to start debating the bill which makes it easier for workers to be sacked in their first 90 days of work.
Maori Party co-leader Pita Sharples said there was no evidence the proposal provided any gains for new or marginalised employees.
Dr Sharples said it was an area where his party would be disagreeing with National as allowed under their confidence and supply agreement.
The bill will be the second piece of legislation put up by the Government of which Mr Sharples and fellow co-leader Tariana Turia are ministers outside Cabinet.
The Maori Party's votes are not crucial to passing the legislation, which the Government intends to do by Christmas.
It has yet to make public copies of the bill.
The Maori Party's employment spokesman, Hone Harawira, said the bill was essentially the same as the bill put up by National MP Wayne Mapp in the last Parliament.
"Maori workers, workers generally are going to be adversely affected by this bill," Mr Harawira said.
Prime Minister John Key said it had campaigned hard on the issue and it was important to get the "growth enhancing" move in place, when New Zealand was facing job losses.
"This will assist the small to medium to medium enterprise sector to take on employees and to take them on rapidly," Mr Key said.
Labour leader Phil Goff said the public had been blindsided and should have a right to have their say on a law that removed basic rights.
"This piece of legislation which takes away those rights is going to be rammed through Parliament with no opportunity for public input," he told NZPA.
"That really is an outrageous act of arrogance and bodes badly for the future of this government that they are prepared to do so."
Mr Goff said the move would undermine workers' sense of security at a time when unemployment was rising.
He said it undercut Mr Key's rhetoric around providing security for workers in tough economic times.
The media are yet to see the bill and Mr Harawira said he had received an analysis of the bill this afternoon, but had only seen parts of it so far.
He did not think the disagreement over just the second piece of legislation brought forward by the Government would hurt the relationship.
"We have an honest relationship with National, we are obligated to support them on confidence and supply, but we are free to make our own decision on all other pieces of legislation."
The bill would give businesses with fewer than 20 employees the right to instantly sack new staff.
The new bill has caused an outcry from unions and the Greens and Labour have urged the Maori Party to oppose it.
The surprise move to pass the law under urgency will mean the public and groups such as unions won't have an opportunity to make submissions on the bill.
Unions said the change, which gives bosses a 90-day grace period to dismiss new workers without exposing themselves to claims of unjustified dismissal, would strip about 100,000 workers at any one time of a basic employment right.
But Labour Minister Kate Wilkinson said the public had already had a chance to give its views when the Mapp bill was considered.
Ms Wilkinson told NZPA said the Government had taken several concerns on board and come up with a "gentler" bill.
Key changes included:
* barring employers from using the law to keep staff permanently casual by sacking them within 90 days then rehiring them;
* removing the benefit stand down period for those "let go" after the 90-day period;
* making mediation available for employers and staff who experience problems;
* making it clearer that workers will still have the right to lodge personal grievance claims on the basis of sexual harassment or discrimination.
The legislation was not on the list of legislation National said it would pass in its first 100 days in government.