Cleaners, including those who work at Parliament, have told MPs a proposed law change will add more stress and insecurity to their lives.
Parliament's Transport and Industrial Relations Committee has been hearing submissions on changes to the Employment Relations Act.
One of them would allow companies with fewer than 20 workers to be free from a requirement to keep workers on when contracts change hands.
The Service and Food Workers Union says cleaners would be particularly vulnerable as cleaning contracts are frequently won and lost.
Parliament cleaner Mareta Sinoti said the change would increase uncertainty about her job.
"We already have really low pay. Don't make this change and take away the few rights we still have - the only thing that protects our job," she said.
Meanwhile, the Dairy Workers Union says removing the obligation on workers and employers to reach agreement on collective contracts does not make sense as the whole point of bargaining is to get agreement.
It says the change will lead to take-it or leave-it deals being put to unions.
It also opposed other changes, such as partial pay reductions during partial strike action and exemption from coverage by collective agreements for a worker's first 30 days in a new job.