The US IRS puts out an annual report on the top 400 US taxpayers. In 2007, the average federal tax rate paid on adjusted gross income was only 19% despite the existence of the alternative minimum tax and a top US marginal tax rate of 35%. This is because 2/3 of their income came from capital gains.
This table shows the effect of the Clinton tax increases and the Bush tax cuts:
From 1993 to 2002 significant numbers of the top tax payers paid an effective tax rate of greater than 35% but none did before or after. The numbers paying less than 15% increased significantly in the later Bush years. Warren Buffett famously said that he paid a lower tax rate than his secretary. At least I'm not paying a higher rate than these billionaires :) Though Snork Maiden is :( Of course, those US figures don't include state taxes, which don't exist in Australia. But they'd likely only add an extra 5 percentage points at most despite so many wealthy people living in high tax California and New York.