I agree with Mr. McKinnon. This is an empirical question, so there's experience on which to base an answer. The answer is that about 20% or 25% would not answer if the survey were not mandatory.
Let me say one other thing. The key thing is identifying the mandatory census as an aspect of citizenship. Everybody has talked about the cost, but the notion of the mandatory census is that it involves a commitment to your fellow citizens. You're providing something at your cost, but there is a community benefit.