Union workers will have to hold secret strike ballots under a law passed by parliament on a vote of 61 to 60.

Under existing law they can choose to have open votes on a show of hands or hold secret ballots.

The new law comes into force in a year's time, and after that unions will have a further 12 months to change their rules.

After that they won't have a choice and opposition parties say that's unwarranted interference.

The Employment Relations (Secret Ballot for Strikes) Amendment Bill is not a government bill.

It's a member's bill drafted by National MP Tau Henare, who says unless secret ballots are enforced workers can be intimidated into voting for strike action.

Government MPs supported it, shouting "it's liberation day" when the bill passed was passed into law by a single vote.

Labour strongly opposed the bill, saying most strike ballots were secret anyway and the legislation was unwarranted.

"They think it's really smart to pass this bill, which is 1970s mentality," said MP Andrew Little, a former union boss.

"This is about increasing the domination and control of employers, workers should be able to make their own decisions about ballots."

The Greens said the legislation meddled with the right of unions and workers to decide how they wanted to decide on strike action.

NZ First leader Winston Peters said Mr Henare had once been a union organiser.

"Now he's turned on those he once represented, he's a sell out."