On me asking for a copy of my job description, our HR person also pointed out this line in my terms and conditions. "Please see attached your signed T&C’s. As you can see under the “Job Title” section, it refers to “from time to time you may be required to undertake duties within your capacity, by your manager”."
I've made the point that ongoing responsibilities that don't have a determined end date don't fall into a definition of 'from time to time'.
Trying to avoid assigned work by parsing your job description with HR is a mistake. Clearly, you are expected to do the work that is assigned to you.
And expecting that your permission is required for your the details of your role to be changed is also a mistake.
You should discuss this with your manager.
Talk about your role. Ask about what your priorities should be. Talk about how you should be allocating your time among tasks. Ask how much longer you will be expected to help with the new task.
Talk about your value to the company and how it will expand along with your expanded role. And if you sense that it is appropriate in the moment, ask what you need to do to get a raise.
And of course if your really don't want to do this additional task, or you feel you aren't going to be adequately compensated, find a new job elsewhere.