Breach of Contract
In an employment context a breach of contract occurs if either the employee or employer fails to discharge one or more of their contractual obligations. For example, an employer could breach the contract by imposing a reduction in pay without the agreement of the employee, or an employee could breach the contract by refusing to carry out an essential duty or disclosing confidential company information to a third party. A fundamental breach to the root of the contract by an employer may entitle an employee to resign claiming Constructive Dismissal. A breach on the part of the employee may justify disciplinary/dismissal action by the employer depending on the seriousness of the breach.
LRA - Advisory Guide - Advice on agreeing and changing contracts of employment