Unions will be legally bound to carry out a secret ballot to approve strikes after a bill sponsored by National MP Tau Henare was approved by one vote in Parliament.

The private member's bill passed its third reading this evening with 61 votes in favour and 60 against.

Mr Henare, a former organiser for the Clerical Workers' Union, introduced the bill to allow members to make a vote on striking without pressure or intimidation from bosses.

"It is about making a decision without the sword hanging over the top of you.

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"Is it too much to ask for that workers get the right when they are asked to withdraw their labour to do it in such a way that is secret to them?"

He argued that it was not an attack on unions because many of the largest organisations already held secret ballots and his Employment Relations (Workers' Secret Ballot for Strikes) Amendment Bill would seal the initiative in law.

In his time as a union organiser, Mr Henare had taken part in show-of-hands votes where he felt leaders could influence the vote just by being there.

National, United Future, and Act backed the bill. Labour, Greens, New Zealand First, the Maori Party and Mana opposed it.

Labour's labour spokeswoman Darien Fenton said the legislation change was frivolous because most unions allowed secret ballots: "[Union members] have freedom to strike or not strike, they are not

marched out the door with their arm behind their back and told to strike."

Big unions such the 14,000-member Post Primary Teachers' Association are among those that already hold secret ballots.

Mr Henare said even if his bill was for the 5 per cent of workers who didn't have a secret ballot, it was important.

Green Party MP Denise Roche felt the bill meddled with a process that did not need to be fixed. "This is a solution looking for a problem."

She argued that Mr Henare's bill intervened on the way unions functioned and was therefore undemocratic.