Secondly, “volunteer” is not defined anywhere; we don't define the term “volunteer” provincially, federally, or anywhere at all. There are some internal documents within the ministries through which they have an idea of what a volunteer means: that they're doing the work for altruistic purposes; that it's not to advance their own career.
But you're absolutely right that the line is murky between the idea of an intern and an employee, in some cases, whereas the terms student or co-op are defined, in large part. But an internship can be a co-op; a student can do an internship. Many of these definitions overlap. You can have a paid internship, in which case the intern is an employee.