Hi Aldo
I concur with Ozboy.
I don't know the exact percentage but I've read claims of 40%. That sounds too high to me and I think it is a misquote. I think the 40% could be a reference to the idea that a significant proportion of migrants who do not stay in Oz leave within 2 years of arrival.
I've also heard that a high proportion of those who do not stay in Oz are Brits because we have a much higher standard of living in the UK than is the case with emerging countries. If the Aussie Dream doesn't work out, coming home will not be a disaster. Plus we have less of a "grin and bear it" culture than people from, say, China. It could be that statistically Brits have higher expectations of Oz than people from many other cultures. I think there are probably lots of different factors involved.
DIAC and other Government agencies in Oz do keep fairly detailed stats about this topic. Some of the figures can be accessed via the link below. I think there are State figures as well and also they look very closely at the cost of providing health care for the migrant intake, for instance. (Plus education, social security etc I imagine.) I suspect that a statistician in Oz will never be short of a job!
Overseas Arrivals and Departures Statistics - Statistics - Publications, Research & Statistics
Cheers
Gill