When you enter into a contract of employment, the contract (theoretically) runs forever.
This is clearly not practical and often contracts include a termination clause allowing for the contract to come to an end in certain circumstances. All employment relationships can come to an end where an employee commits and act of serious misconduct such as theft (poor performance is not enough).
In any other circumstance the employer can end the relationship by providing the amount of notice contained in the contract or award. If there is no termination clause contracts contained “implied” notice terms that can be quite large.