The absolute destruction of houses down to twisted metal and ash is not caused by the bushfire itself, but by the burning contents of the house. 

Often the vegetation near the house is untouched by fire.

When an unattended spark or ember from bushfire is blown inside a house, it smoulders, flares, and fire spreads through furniture and furnishings, clothes and kitchen contents, papers and plastics and fly-sprays and cleaning fluids until only ash and twisted metal remain. People need to be aware which aspect of houses lead most to the most destruction: not the cladding, the contents!