Prof Brand-Miller's GI endorsement program earns a percentage of the sales of CSR's Low-GI sugar (and a long list of other high sugar foods).
It think the Statistic she cites about declining sugar production comes from a study done by Alan Barclay (her Chief Scientific Officer and occassional acting CEO). The study has not been published and he has ignored my requests for a copy.
The most reliable data I can find on the point is from the Australian Bureau of Agricultural and Resource Economics.
When you put ABARE's Australian domestic sugar availability numbers on a graph, they look like this.

Sugar availability (for domestic consumption) is highly variable. But its obvious that if you are very careful about picking which peak and trough you compare, you could certainly conclude that consumption has dropped between (say) 1978 and (say) 1999. Its also obvious that the trend line tells a very different story.
These numbers come from http://www.abare.gov.au/publications_html/ac/ac_10/ac10_June_a.pdf
Cheers
David.