Is it acceptable for the secretary to have full access to our entire Outlook agenda? Today we were asked by the management of our department to give full access of our agenda in Outlook to the secretary. By full access, I mean the possibility to look, read, write, delete (own) slots in the agenda. It was mentioned that it's imperative not to alter any settings so the entire agenda will be visible to them.

The rationale is that sometimes people come in the department looking for us and by giving full access of our agenda, the secretary can advise these people on when/how to contact us. 

On a personal note, I have never experienced something like that. People who want to find me can call me or send me email. I do not see why I should give access to my Outlook agenda, where I also store personal events.

So my natural question is: Is this normal/acceptable? How can I
politely say that this, in my opinion, is an unacceptable practice
possibly violating privacy?

My department is located in the Netherlands for what it's worth. No prior communication of this problem has been done/discussed.
My own perspective on this from the UK is that it is completely normal outside universities that outlook calendars are made fully accessible by anyone in the org. Many of my non-academic friends continually express surprise that I am allowed to keep my calendar private (I just share busy/free information). Even within universities, all of my professional service colleagues are expected to make their schedules accessible. On the other hand, this would seem to be a place where in general academics tend to be privileged, and I've not heard of faculty colleagues who have been forced to make their calendar public.