Employment Relations Act 2000, ss 7–11
As an employee you have the right to:
join a union
decide which particular union you should join (out of those unions that cover the type of work you do)
not join a union, or resign from a union.
Your boss can’t pressure you into not joining a union, or treat you unfairly (“discriminate” against you) because you’re a union member.
Employment Relations Act 2000, s 110
You can take a legal claim (a personal grievance) against your boss if:
they tell you that you can’t belong to a union if you want to keep your job, or
they use threats or incentives to make you quit the union or to stop you representing other workers in that workplace.