Part 6A of the Employment Relations Act was introduced by the former Labour Government in 2004.
It means businesses with fewer than 20 workers that win a new contract have to retain cleaners employed by the previous contract holder.
Former Minister of Labour Kate Wilkinson proposed changes to the Act last October which would mean those companies can get rid of workers they have inherited. Her successor, Simon Bridges has taken up the policy.
The Services and Food Workers Union has deemed those workers "vulnerable" because such contracts often change hands.
Lalopua Sanele, a cleaning supervisor for Spotless Services at Wellington Hospital, said she was angry about the proposed changes because she fought hard for changes that became Part 6A of the Act.
"The National Government are not thinking hard before they talk about this law - some of the cleaners here are working two to three jobs a day."
Labour Party leader David Shearer told the group his party would repeal any changes if it wins next year's election.
Grant McLauchlan, managing director of franchising company Crest Clean, supported the changes. He said Part 6A created vulnerable businesses, and employers should be able to decide if they want to keep staff.
The proposals have passed a first reading in Parliament and will go before the transport and industrial relations select committee after submissions close tomorrow.
Mr Bridges said he would listen carefully to submissions.