I work in admin at one of the universities in the city. Recently, one of my colleagues left the department for a new job. It was expected that I would take on some of their workload. This was not something I could really do, and when I started to be asked, I found it stressful. I talked about it at the drop-in, and a number of useful suggestions were made.
I started to say no when I was asked to take on new tasks. I also started working slower, and I deliberately got something wrong, in order to highlight how high my workload was. My boss stopped asking me to take on any more work, and within a week or so of doing this, an interim administrator had been appointed, whilst the department advertised for a replacement. The department had said that they were going to do this at some point, but I’m sure that it happened much quicker as a result of the action that I took! They had also not said how often the interim administrator would be working, but they have been appointed to cover all of the hours of the previous administrator, meaning that their workload is not being put on me or anyone else in the department.