The Council of Trade Unions says the National Party's new employment and workplace relations policy would reduce workers' rights.
The policy, released on Thursday, includes the ability for employees to cash in their fourth week of annual leave.
National has already flagged its policy to give employers the right to dismiss staff after a 90-day probationary period, for businesses with fewer than 20 people.
The party says it will review the minimum wage, and says it will not stay static.
Under its policy, National would allow workers to bargain collectively without belonging to a union. It would also appoint a working party to review the Holidays Act.
Council of Trade Unions president Helen Kelly, says the policy would mean new employees would be treated unfairly.
However, National says it would put limits on the proposed probationary period, would not restrict union access and would work to increase wage levels.
National says it recognises there was no desire to return to the Employment Contracts Act of the 1990s and it believes everyone will be treated fairly under this policy.
The Labour Party says the policy will constrain wage growth and shift the balance of power from employees to employers.