When you have to select the Proficiency of your English, you will have 4 choices.
If you select High Proficiency, you have to prove that you really are.
There are two ways to prove.
#1 Either you are born and raised or raised only in an English speaking country for example NZ, Australia, UK, USA and Canada etc. In this case you have to show that you did your high school and college in one of these countries.
#2 If you do not qualify for #1 and you choose high Proficiency then you have to take IELTS and you have to get very good score.
If you choose Moderate Proficiency, you don't have to take IELTS. They will know your when Proficiency you take your interview.
I am not saying all these on my own; I attended a workshop about immigration provided by Canadian embassy in NY.
Here is the link to site to take quiz, you must get at least 67 in order to be sonsidered yourself eligble.
http://www.cic.gc.ca/english/immigrate/skilled/assess/index.asp
just click on