Ms Fenton also referred to "vulnerable workers" such as cleaners and kitchen workers, whose employers changed every two to three years.
Under the current law, the new owner is bound by existing contract conditions, which was expected to change, Ms Fenton said.
"What they are proposing to do is make it so only major employers are covered by that protection . . . the problem with that is most cleaning companies or employers have fewer than 20 staff."
If Labour got into power next year, the first thing it would do would be to repeal the changes, Ms Fenton said. "They are unfair and not needed. I find it hard to understand why the Government would want to do it. It's already tough for workers out there."
Minister of Labour, and Tauranga MP, Simon Bridges said the changes in the bill were about increasing flexibility and fairness in the labour market, which in turn would increase productivity.