Finally finding some stats on HDTV penetration although they're not very good stats.
At Sep 06, 23% of homes use free-to-air digital TV.
http://www.dba.org.au/newsletter/IB-DecJan07-full.asp#WEBSITE
Of course, not all these people have HDTVs.
275,000 plasmas and LCDs were sold in the December 2006 quarter.
http://www.smh.com.au/news/home-theatre/aussies-choose-lcd-tvs-over-plasmas/2007/03/16/1173722711373.html
So that's would mean maybe around 1,000,000 per year including other types of HDTV. So as a percentage of the 7.6 million homes, that's 13.15%.
Previous year same quarter was 77,731 plasmas and LCDs which is 300,000 per year. So maybe that would mean 4%
Can't find any other stats but I would guess that maybe 20% of homes have a HDTV based on that.