Hello my name is Greg. It looks like your other expert is offline and this information is urgent so I am going to help as much as I can for you.
The most recent data is from the 2011 census as the 2012 data has not been released yet.
According to the official census data from Australia, the number of Australian that are of Italian descent is 916,121 or 3.3% of the population as compared to 852,421 or 3.3% of the population in 2006 census data. The numbers are not changing too much although with a higher population of Asian descent coming into Australia, this will begin to lower, in percentage, over time.
Of the population that was actually BORN in Italy and now reside in Australia (as opposed to ancestry numbers above), that would be 185,402 or .9% of the population (per 2011 census data). I am not sure which number you require (ancestry of 3.3% or born in Italy of .9%)
People — cultural & language diversity and you will find those sections below.
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All the best in your studies and have a wonderful rest of the week.