New rules are to be imposed on union members when they consider taking strike action.
Parliament's passed a Private Members' Bill that will now require such matters to be subject to a secret ballot.
The Bill's drafter, National MP Tau Henare, says it's about democracy.
"If it's good enough for your nation to have a vote by secret ballot every three years in a general election, why isn't it good enough to have a secret ballot when somebody is withdrawing their labour?"
Labour MP Darien Fenton says the law change is unnecessary.
"They have freedom to strike or not strike, they are not coerced, they're not marched out the door with their arm behind the back and made to strike."
National MP Jami-Lee Ross says the Ports of Auckland dispute probably wouldn't have got as bad as it did if there'd been secret ballots.
"They had people there intimidating them, coercing them into voting for strike action, they weren't able to make their own mind up."
Mr Ross says dozens of workers have since left the union and returned to the waterfront because they want to work.
The Bill passed 61 votes to 60.
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