The National Party says the Government has been sneaky by slipping in an amendment to Kiwisaver as part of a bill amending the Employment Relations Act.
The bill, enforcing rest breaks and facilities for breastfeeding at work, passed under urgency in Parliament on Wednesday.
During the committee stages of the bill, the Government also put up an amendment making it unlawful for an employer to offer a reduced salary package if an employee is a member of Kiwisaver.
National says the bill has not been subject to proper scrutiny, and has been sneaked through at the eleventh hour.
Labour Minister Trevor Mallard says the Government had left the issue of Kiwisaver contributions to good faith, but some employers were taking advantage of the situation and the law had to be changed.
Mr Mallard says the change is small but important, and organises the law in the way that most people had thought it was supposed to be working anyway.