Some resources here for you to look at.
http://www.bizhelp24.com/business-law/employme nt-contracts-13.html
Usually the job title and a summary description ONLY form part of the employment contract though. It doesn't help either side to try and make a 'job description' a complete statement of what the employee will do - the world doesn't work like that. More detailed job descriptions are typically required for job evalaution processes, say.
If a job description is within an employment contract the employer will put a caveat on it like 'any reasonable task in line with the knowledge and skills of the person'. Any arguments then will revolve around the 'reasonableness' of an additional task.
What is your employer seeking to change?