Extending visitor is 6 months at a time.
Even though you can't extend a WHV, you can apply for it again and keep applying for it again as long as you are under 30 so that might be a better option. You do not have to go to Australia to do it, it's enough to leave Canada, for example go to the US. Check out
http://www.whpcanada.org.au/faq_eligibility.shtml#eligibility_q15 question 15.