In January, PM Jacinda Ardern and Workplace Relations and Safety Minister Iain Lees-Galloway announced moves to restrict the use of 90-day trials.

Labour's flagship workplace relations bill has been delayed and union access provisions softened to keep NZ First on board.

The rights of employers to opt out of multi-employment collective agreements (MECA) has also been strengthened, with employers able to opt out of concluding them if they have "reasonable grounds".

The changes to the bill appeared in an amendment put forward by Workplace Relations Minister Iain Lees-Galloway ahead of the bill's second reading in Parliament this week.

It's understood these changes were made after negotiations with NZ First, after the party expressed some discomfort in September when the bill emerged from select committee.

The bill is the Government's flagship workplace relations policy, and mostly takes the law back to where it was when Labour were last in Government. It includes provisions to end the 90-day trials for all employers with more than 20 staff, re-introduce the right of rest and meal breaks, and make life much easier for unions attempting to bargain with employers.

TOM LEE/STUFF Minister for Workplace Relations Iain Lees-Galloway and Prime Minister Jacinda Ardern confirmed they had all three parties of the coalition on board with any changes made to the law.

One of the main changes is to union access to workplaces. The first version of the bill had ended any need for union officials to get consent from employers to access workplaces - this has been quashed, although officials will still be able to enter any workplace where they already have a collective agreement established, or are bargaining for one.

This was one of the big bugbears of regional businesses who NZ First are looking to protect.

The bill's enactment has also been somewhat delayed - instead of being enacted four months after royal assent (likely March next year) it will come into effect on 6 May 2019.

A small change has also been made to the rest and meal breaks but this is more technical. The changes to the MECA opt-out were also more of a clarification than a backdown, as the Minister has long said that employers should be able to opt out if they had a good reason to.

Lees-Galloway and Prime Minister Jacinda Ardern confirmed on Tuesday morning that they had all three parties of the coalition on board with any changes made to the law.

NZ First already won a concession on the bill prior to its first reading - Labour had wanted to end the 90 day trial for all workplaces, but backed down into ending it for only workplaces with more than 20 employees.

National and the business lobby fought an extended campaign against the bill, and NZ First's support looked to be wavering when it returned from select committee with few changes in September.

Party leader Winston Peters notably called it a "work in progress," while NZ First MP Shane Jones bagged it in an interview with Q+A. It's understood this interview is was set the behind-the-scenes political turmoil off.

Prime Minister Jacinda Ardern said the bill was the result of "constructive internal discussions."

It's understood NZ First chief of staff Jon Johansson negotiated with Heather Simpson, the high-powered former chief of staff to Helen Clark. The negotiations wrapped up about a month ago.

"Each party has bought their different views and the subsequent consensus reached will result in employment law that is better for workers and employers," Ardern said.

"It's an iterative process when you design policy as part of a coalition."

"Every aspect of our Bill is better than the current employment legislation and delivers rights and protections for workers which were stripped from them by National. Overall there is much to celebrate in this legislation, which I am sure will lead to better and more consistent outcomes in New Zealand workplaces."

Ardern said she was confident the Labour Party's affiliated unions could live with the bill.

Peters said the changes were a "serious victory for the economy" and the result of "common sense".

"It means we are going to have durable law reform in the industrial relations area that can last beyond this Government," Peters said.

"You can't always get what you want. But you can get as much as you possibly can."

Council of Trade Unions head Richard Wagstaff said the bill was not everything unions wanted but they "had to accept the reality of coalition Government."

"It's delivered on what we wanted in terms of restoring the law to largely where it was [before National came to government,]" Wagstaff said.

He was not particularly worried about the softening of union access provisions as "in the large majority" of union access disputes a collective agreement was already in place or being bargained.

Wagstaff was clear that the bill showed MECAs were not likely to be greatly extended as a result of the bill, but he hoped that over time New Zealand could move to a multi-employer bargaining framework.

BusinessNZ welcomed the changes, but said they still had concerns with parts of the bill.

"The ban on larger businesses from using 90-day job trials remains, and also the requirement for employers to reach agreement on bargaining demands for collective agreements, a provision which in our view would breach international law," BusinessNZ head Kirk Hope said.

National Party workplace relations spokesman Scott Simpson said his party was still dead set against the bill.

"This Bill forces extra costs on business, allows pay rates to be settled elsewhere and lets union officials enter workplaces unannounced," Simpson said.

"The coalition's junior partner, NZ First, has failed to get any substantial push-back on what is an ideologically driven Bill. Given it wrangled a $3 billion slush fund out of the coalition agreement, the public must wonder what horse-trading went on to tame Winston Peters."

"Business leaders say they were ignored during the passage of the Bill and may have been hoping NZ First would argue on behalf of firms both large and small. But it has achieved little for the SMEs it purports to care for, which will now face increased costs they can't control."

He promised that any future National government would repeal the law.