r/ Sydney, Is my employer allowed to do this?

long story short; I work in a large public institution and I am a pleb as far as management are concerned, recently they changed the working conditions in a way that impacts employees' health. Now we are attempting to appeal that the decision has negatively affected us and our ability to do our job properly and as often (so I've gone from 30 to 40 hours a week to 20 to 25). But rather than listen and act on our concerns they are suggesting that if we are not fit to do the job then we should reconsider our position, and they have more or less insinuated that any staff who don't just accept it will be let go.

Before the change there was no consultation until we called a meeting with them, and their entire response has been to treat us as if we are whinging and our concerns are false. Essentially management changed our working conditions without any consultation and are convinced that their actions are justified, they then refuse to listen to reason and staff negotiation. What can the employees do?