Oct 2012
Between July last year [2011] and June this year [2012], about 19,540 people were found to have stayed in Australia for longer than their visa allowed -- more than double the number of asylum-seekers who arrived in the country by boat during that period.
The Australian
2012 - 17 202 boat passenger arrivals (Govt statistic)
2013 - 20 587 passenger arrivals (source news.com.au)
As mentioned above all overstayers are counted in the overstayer statistics even if they leave the country a day, a week, or a month etc later than the date their visa expired.
