I'm not a society specialist. Not yet. Won't be able to give specific data or analyze for the multicultural situation in Australia now.
So I'll just give my personal humble opinion.
Multicultural is a good and hot topic. Not only in australia, but international. We all know multicultural has brought a lot of disputes and some conflicts in many countries. Even the preserved and traditional China nowadays starts to face this issue. There are thousands of fights or even wars, just for figuring out which cultural is right. Not only today, but from the start of human civilization.
Religion, has long history and has very fundamental effect in human behavior and cultural. I'd like to give religion a small talk. Some people believe that religion is the incentives of wars and conflict. True fact is any beautiful slogan can be used by dictators to manipulate citizens to encroach and plunder other nations. Any thing could be propagated as the reason when conspirators want the citizens to join the war. Religion, peace, wealth, equality, stronger, better and etc, which give those dictators more leniency in hyping the atmosphere, unfortunately often been used. To judge it wisely, we need to learn more about the issues behind the wars.
Back to monocultural, from the positive side, it won't bring conflict. Every one will be the same, then no point to fight for who is right, who is wrong. but, this is too idealistic. First, there's no real monocultural. Let's say west Europe cultural is the main stream and we make all Australians become completely western Europeans. As main cultural and religion in west Europe at present, as well as in Australia, would be be Christian and Atheism. Then other problem comes. In order to be monoculturalism, should we choose Atheism or Christian? If we follow the majority Christian, then all different christian branches will have a new debate. Which one is the pure orthodox Christian religion branch? Then we may need to choose Catholic. How many of us would want to become pure Catholic? Would the whole society become peaceful from then on? Of course not, have a look at the historical Britain in dark ages.
So, in my opinion, monoculturalism is a history back forward. Multicultural is a big symbol of human civilization. Yes. It brings problem. But what we really need to figure out how to implement multicultural better rather than push the history wheel back forward.