A union campaign has been launched opposing the extension of the 90-day trial period for new workers to all employers.
The trial period previously applied to businesses with fewer than 20 employees and the extension is among changes to the Employment Relations Act which takes effect on 1 April.
The amended act also requires unions to get permission from employers before entering workplaces and allows employers to ask workers to provide a medical certificate for a single day off sick.
In addition, employees will have the right to cash in their fourth week of holiday entitlement.
The Engineering, Printing and Manufacturing Union's national secretary, Andrew Little, says the changes are unfair.
He says the union is recommending to members who are in bargaining this year that they raise as a claim "clauses that mean that the employer won't or can't rely on the new law change, as that's perfectly lawful."
The union will campaign under the slogan "Things just got tougher".