Opposition parties say employment law changes due to be introduced in the middle of the year are emboldening employers to be more militant.
Labour Party industrial relations spokesperson Darien Fenton says it's clear companies are taking a much tougher approach in negotiations with their workers as they look ahead to the likely employment law changes.
"Definitely some employers have become emboldened by the Government's intentions to weaken collective bargaining,"
But Labour Minister Kate Wilkinson says the Government cannot be blamed for industrial disputes at the Ports of Auckland and meat company AFFCO.
She said the timing of industrial action often depends on when collective agreements come up for bargaining.
Ms Wilkinson said workers have nothing to fear from the proposed changes.
She says Labour has criticised the 90-day trial for new employees but a recent study estimated that has helped 13,000 people into jobs.
Mana Party leader Hone Harawira says that as soon as employers think they can put pressure on workers, they do.
Industrial relations specialist Dr Stephen Blumenfeld of Victoria University says National signalled during the election campaign that it would make changes to employment laws the unions would not like.
However he says employers had already started to "test the waters" following the enactment of the so-called Hobbit law.
"That was where the Government stepped in and changed the employment laws for one specific industry; changed it in a way, obviously, that unions are not happy with."