Q. What causes bushfires in Australia?

Short answer:

Lightning is the cause of almost all naturally occurring bushfires”. “Human activities account for most of the rest with accidents associated with burning off, campfires and machinery … [and] between 25 to 50% of bushfires are thought to be deliberately lit”.

Causes of bushfires in Victoria – ABS, 2004

A 2004 ABS study found that these were the causes of bushfires in Victoria:

Causes of bushfires

Data collected between 1976-77 and 1995-96 on the causes of bushfires on public land in Victoria shows that lightning strikes lead to the highest number of fires each year, followed by fires that are deliberately lit, those that escape from agricultural burning, and escapes from campfires. Similarly, the majority of area burnt in Victoria over the 20 year sample period, was initiated by lightning, followed by public utilities (e.g. from power lines), and deliberately lit fires.

Source: ABS, 2004 Year Book Australia

In 2010, a Senate Select Committee investigated Bushfires in Australia:

The incidence and severity of bushfires across Australia
Incidence:

1.19 CSIRO noted that bushfires ‘are an inevitable occurrence in Australia’. “About 50 million hectares of land are burned across Australia each year on average and about 80% of fire-affected areas are in northern savanna regions. Lightning is the cause of almost all naturally occurring bushfires”. “Human activities account for most of the rest with accidents associated with burning off, campfires and machinery being the most common sources of ignition. While it is difficult to assess the magnitude of maliciously lit fires, between 25 to 50% of bushfires are thought to be deliberately lit”. “Bushfires account for about 10 percent of the cost of all major natural disasters in Australia, and are associated with the greatest loss of life”.

Severity:
1.20 They [CSIRO] described the different nature of fire regimes across Australia: “Fire regimes across Australia vary because of variation in the rate of vegetation (and hence fuel) production, the rate at which fuels dry out, the occurrence of suitable fire weather for the spread of fire across the landscape, and ignitions … Regional fire regimes differ because of variation in one or more of these key drivers. As a consequence, fire regimes in some areas are constrained primarily by availability of fuel, in others by the occurrence of periods of suitable weather. For example, the tropical savannas of the north tend to burn mainly in the winter-spring period and experience high frequency and relatively low intensity fire regimes … In contrast, the tall sclerophyll (eucalypt-dominated) forests of the cool, temperate south tend to burn in summer and generally have low frequency/high intensity fire regimes.
Source: The Senate
The majority of bushfires in southeast Australia are caused by human activity
The majority of bushfires near populated areas are the consequence of human activity. Lightning causes the smaller portion naturally. Sometimes, a carelessly discarded cigarette or a glass shard, which can focus the sun’s rays is all it takes to start a fire. Heat from motors or engines, or electric sparks from power lines and machines can ignite dry grass. Besides this [these] accidental causes, a significant share of wildfires are started deliberately. Humans also change the natural fire frequency and intensity. They decrease the natural fire frequency due to deliberate fire suppression near populated areas. If there is no fuel-reduction burning in forests for the purposes of fire prevention, large quantities of combustible material can accumulate at ground level. Surface fires in these areas can become so intense due to the large amounts of fuel that they spread to the crowns of the trees and rapidly grow into a major fire. If humans had not intervened in the natural bushfire regime, more frequent low-intensity fires would have consumed the forest undergrowth and ensured that woodland grasses and scrubs do not proliferate excessively.
Source: Munich Re