Thanks to information provided by clath from homeone.com.au forum, I found this article titled "Downlights fire warning" from Channel 7 Today Tonight web site talking about the danger of downlight setting fire to insulation in the roof.
There's even a video on the web site.
"Ceiling downlights are a serious fire risk thought to cause at least one home fire per week by igniting roof insulation. And because they burn in the roof cavities above fire alarms residents only know their house is burning when the ceiling collapses."
"Metropolitan Fire Brigade investigators Rod East and Colin Fowler say over the past 18 months, in Melbourne alone, more than 57 houses have been damaged or destroyed by a downlight fire."
"Electrician and Managing Director of A and F Electrical, Frank Spiteri, blames a "do it yourself" mentality sweeping across Australia, for an increasing number of dodgy installations."
I guess it could be caused by the downlights not been insulated/installed properly, or the insulation not been installed properly, or both.
If you have downlights in your house, you might want to get a licensed eletrician to check if it's correctly installed!
If you found any problem and looking for solution, thanks to Mark/Cookiemonster from homeone.com.au forum, the following 2 products would certainly help:
(1)Isolite - Downlight Guard 94-114 for $14.95 each.
(2)Isolite Transformer Isolator for $9.95 each.