The Outdoor Power Equipment Association is about the only crowd who collect these sales stats by units sold. As they only publish these to their members, the only public access to this data is that which makes its way into government reports.
The most recent stats available by this route are for FY's 2002-2009, and these list new petrol rotary mower sales from a low of 223,609 [2002-3] to a peak at 421,000 [2007-8]. So much in the same ballpark, really.
The full document is
HERE - 5.3MB PDF of an OPEA submission from 2010.
