What it means
Ultra vires means acting "beyond one's powers". An act is ultra vires if a person, company or government body does something the law or its governing rules do not authorise. In Australia the concept arises with companies acting outside their constitution, and especially with public authorities and delegated legislation: if a regulation or decision goes beyond the power granted by Parliament, a court can declare it ultra vires and therefore invalid.
How it's used
The council's new fee was struck down as ultra vires because the legislation gave it no power to charge it.