Sorry, forgot the link
http://www.oecd.org/australia/pisa-2015-australia.htm
You can look up the detail yourself if you're so sceptical. I think you'll find PISA(Programme for International Student Assessment) is fairly highly regarded.
"There is a steady decline in the results since 2000, both in terms of overly simple international comparisons and absolute mean scores."
http://www.abc.net.au/news/2016-12-07/pisa-australia-ranks-poorly-but-what-can-we-learn/8097546
