The Government says it will torpedo Labour MP Sue Moroney's bill extending paid parental leave to 26 weeks

The Government has confirmed it will use its power of financial veto to stop a bill extending paid parental leave to 26 weeks after it failed to stop the bill passing its first hurdle in Parliament.

Labour MP Sue Moroney's paid parental leave bill passed by one vote on Wednesday to allegations of Government dirty tricks designed to thwart it.

National opposed the bill on cost grounds but can only stop it now by wielding its power of financial veto, or by persuading the Maori Party or United Future leader Peter Dunne to change their vote.

A spokesman for Finance Minister Bill English confirmed after the vote that if the bill got through to its third and final reading the Government would be required to use its power of financial veto as the policy had not been budgeted for.

The Government puts the cost at $150 million a year. Moroney says the cost is more like $285 million over three years.

The third reading comes after the bill has been considered by a select committee.

Wearing the jacket she wore three years ago when she launched her battle to extend paid parental leave to 26 weeks, Moroney said the Government had pulled "every mean trick" in the book to thwart her bill, including exaggerating its cost, breaking its promises to her, denying voting rights to an MP who attended the birth of his own grandchild and recording proxy votes incorrectly.

Parliament voted to send the bill to a select committee after Labour gained the support of United Future, the Maori Party, the Greens and NZ First, with only National and Act opposed.

Moroney said her bill recognised that supporting parents to be at home in the first six months of a baby's life was a wise investment in New Zealand's future.

Moroney's bill extends paid parental leave by eight weeks to 26 weeks.

It follows her three year battle to extend the scheme - a battle which has been partially successful after the Government bowed to mounting pressure and announced its own plans to increase the payments to 18 weeks from April 2016. It has also announced extra support for parents of premature babies.

But Moroney has called on the Government to do more, and argues the additional eight weeks' support her member's bill provides gives parents the chance to spend vital weeks with their babies.

"The evidence shows that will support important stages of their brain development. Between 18 and 26 weeks babies are undergoing huge developments such as beginning to laugh, starting to eat solids and sitting up by themselves. All parents want to be at home to share these milestones."

There were angry exchanges during parts of the debate after NZ First MP Tracey Martin took issue with suggestions people should wait to have children till they could afford it.

But ACT leader David Seymour refused to apologise for that stance, saying it was not only what most New Zealanders believed, "It is what the overwhelming majority of New Zealanders do".

National MP Simon O'Connor said there was a sense of "deja vu" debating the bill in the House so soon after it was defeated by parliament.

Moroney resubmitted the bill to Parliament's ballot after the Northland by-election cost National one vote.

National MP Simon O'Connor told Parliament Moroney's sentiments "may be noble but I don't believe them to be correct".

He said the Government was doing what it could by extending paid parental leave out to 18 weeks from April 1 next year.

A Government's responsibility was to all its citizens and while the costs may not be "huge", it had to make choices and reach balance in its decisions.

He also had "philosophical" concerns with backing Moroney's bill, O'Connor said.

"One part of it is that having a child is generally not an accident or a mistake."

People often justified the cost with the explanation that "it takes a community to raise a child" but that was generally "pulled out as an excuse when money is required".