Especially in countries with huge population and economy like Canada and Australia, what's the opinion of people about constitutional monarchy? Are there any well known groups or indivuals who opposes to the form of the government?
According to Wikipedia, Monarchy of Canada is not a subject of active discussions outside of academic circles. Although there is an organization in Canada called "Citizens for a Canadian Republic", founded in 2002, which advocates the replacement of the Canadian monarchy. Wikipedia articles Monarchy of Canada and Debate on the monarchy in Canada has some useful information about the topic.
In Australia, political parties Australian Labor Party, The Greens and The Democrats support becoming a republic. Again, Wikipedia article Republicanism in Australia explains republicanist movements in detail.