The Government has changed its procurement rules so it no longer has to use cleaning companies which are part of an industry association.
Labour Minister Simon Bridges says cleaning companies can now compete on a level playing field, but the Building Services Contractors association believes it could lead to a pay cut for workers.
Before the rules were changed, the Government could only award cleaning contracts to members of the association which has a code of practice covering quality, health and safety and employment conditions.
Mr Bridges said very few, if any, benefits have been achieved for workers by the association.
But the BSC rejects that, saying the rules meant its members had to pay staff an agreed amount above the minimum wage.
Removing the requirement encourages companies to under-cut competitors by lowering wages, it said.