New Zealand First has blindsided Justice Minister Andrew Little by suggesting it may call for a referendum on abortion law reform.

The Government this week unveiled its bill for an overhaul of abortion laws, with the legislation to go to its first reading in Parliament on Thursday.

The bill went through extensive talks with NZ First before being announced to improve its chances of passing, but MP Clayton Mitchell on Tuesday told reporters the party may only support it if goes to a referendum.

"We've had a good discussion about that at the moment and we're working through it but it's something NZ First believes should probably be a binding referendum issue," he said.

NZ First leader Winston Peters wouldn't rule it out.

"This is ongoing work and we'll let you know on Thursday when the vote happens," he told reporters.

"Referendums are what New Zealand First has stood for a long time when it comes to conscience issues. We don't have to raise it earlier. Everyone who has been dealing with us knows that."

But it was news to Little, who has been leading the reforms.

"In all the extensive discussions we've had it's never been raised with me," he said.

"It's a bit unusual it gets raised at this point. We've got the bill drafted as a result of the consultations we've had with them. I'm proceeding what we've got at the moment."

Little said he did not believe abortion was a referendum issue.

He also faced questioned about it from Opposition MPs in the House, answering there was no referendum on abortion had been "mandated or contemplated" by Cabinet.

MP Tracey Martin, who has been speaking for NZ First on the abortion reform, has previously said members would be voting individually on the bill, but declined to comment on Tuesday.

Prior to this term in Government, NZ First and Martin had suggested abortion could be a referendum issue.

It's not the first time Little has been taken by surprise by NZ First.

He also believed he had their support to repeal three-strikes criminal legislation, only to find out, at the last minute, he didn't.

National's Amy Adams, who has been working with Little to get support for the changes among Opposition MPs, was critical of the call for a referendum.

"I think if you're in Parliament, govern," she told reporters.

"Don't be cowards and send everything to referendums."

The bill is expected to pass when it goes to its first reading on Thursday, with some conservative MPs such as National leader Simon Bridges, saying they'd be willing to let a Select Committee consider it.

It is being held as a conscience vote, with MPs voting individually, rather than along party lines.

The most significant change in the law would see the current requirement for women trying to get abortions to get clearance from two doctors on physical or mental health grounds dropped, in favour of open access until 20 weeks' pregnancy.

After 20 weeks, the medical practitioner performing the procedure will have to give approval.

New Zealand First is also insisting on a referendum on David Seymour's End of Life Choice bill, which would legalise assisted dying for terminally ill adults.

A referendum is already being held at next year's election on recreational marijuana decriminalisation.