You have to be careful. Technically you cannot take away work from a Canadian or PR by volunteering - so if you are doing something that a Canadian or PR would get paid for, CIC will see it as not legal. This is because a common way to get out of getting caught working illegally was to say it was just volunteering and the person was not getting paid - this was for jobs such as waiting tables, and the like!
So if it really is something a person would only do as a volunteer, it should be OK. But that is basically because no one would be likely to complain. If it is something that someone might otherwise get paid for, don't do it. And if you are volunteering, I would not advertise the fact that you are not yet allowed to work.