I found 554 entries with matching family and given names.
I had actually expected a higher number of matches. Desmond O’Connor, professor of Italian at Flinders University (Adelaide), in his article From Tobruk to Clare: the experiences of the Italian prisoner of war Luigi Bortolotti 1941-1946, which you can read online or download in PDF format, estimates that 9.4% of Italian POWs held in South Australia returned to Australia as migrants.
If we assume that the same percentage applies to POWs held in all states, we arrive to a figure of about 1,700 returnees (9.4% * 18,420 = 1,731). Further, if we assume, as it seems reasonable to do, that many independent factors influenced the decision to return to Australia, we can apply the Central Limit Theorem (CLT) to estimate the degree of approximation of the mean.
The CLT, for those who are not familiar with it, states that the mean of a sufficiently large number of independent random variables is normally distributed. In practical terms, it means that if we repeatedly measure something that depends on many independent factors and make a histogram of the values we measure, the plot will approximate the familiar bell shape of a normal distribution (thanks to Wikimedia Commons for the following image).
With a normal distribution, we can roughly estimate its standard deviation (indicated with the Greek letter σ in the figure) by taking the square root of its mean. In practical terms, it means that with a mean of 1,731, the standard deviation of the number of returning POWs nationwide is 42. This in turn tells us that, according to O’Connor’s estimate, the actual total number of Italian POWs returning to Australia has a probability of 68% to be between 1,689 and 1,773. And the probability of the actual number being less than 1,605 (3σ below the mean) is less than 1%.
My figure of 554 is so low as to be incompatible with O’Connor’s estimate. Now I have to figure out whether one of us is completely off the mark and why...