The Government is welcoming a new law that will force unions to hold secret strike ballots.
The member's bill was drafted by National MP Tau Henare, it isn't government legislation.
Parliament passed it by 61 votes to 60 on Wednesday night against strong opposition from Labour, the Greens and NZ First.
Mr Henare and his supporters argued that open ballots on a show of hands, which are allowed under existing law, run the risk of workers being intimidated into supporting strikes.
Opponents said nearly all unions hold secret ballots anyway and the bill is unwarranted interference with union rights.
Labour Minister Kate Wilkinson says strikes are stressful, workers have different personal circumstances and it's fair that they should cast secret votes.
The law comes into force in a year's time and after that unions have 12 months to amend their rules.
Ms Wilkinson says she expects them to do that "as soon as practicable".