In south-east Australia, bad fire days are associated with the presence of a 'blocking' high pressure system in the Tasman Sea. This brings hot, dry strong wind from the centre of the continent to the south-east.
The high temperatures, some in excess of 45 degrees, and dry air experienced throughout Victoria on Saturday resulted in very low fuel moisture content. Combined with the extended rainfall deficit for much of the state, this resulted in tinder-dry fuel that was very easily ignited and very difficult to extinguish. In addition, to the high pressure system there was an approaching cold front which helped to strengthen winds ahead of the front, as well as causing a wind change after the front passed. Very strong winds resulted in fires that spread very rapidly with the wind and were practically unstoppable until the weather moderated following the cool change. Victoria's topography and vegetation also played a role.
(A blocking high is a persistent high pressure system that occurs on a large scale, remaining stationary for a period of time, compressing and warming the air below.)