EDIT – Due to the interest, while my previous answer was OK, I’ve summarised official Australian ABC (public broadcaster) Fact Checks on this subject to provide full and correct detail.
The three essentials on how bushfire behaves are: a) weather, b) fuel and c) topography.
In Australia, the Bureau of Meteorology produces ‘fire danger ratings’ in consultation with State authorities. While other risk factors are considered, the Forest Fire Danger Index (FFDI) of 1-100+, a formula developed by fire scientist Alan McArthur in the 60’s, is a major factor.
This index combines measurements of air temperature (why heatwaves matter – indirectly via influence on fuel moisture content and local wind formation, plus the higher the temperature of the fuel, the more easily it will reach ignition temperature), relative humidity, wind speed and fuel conditions with a formula to account for the effects of drought, i.e. how long since it last rained/amount of last rainfall to assess how dry the soil is, and therefore how combustible the fuel is likely to be.
More re air temperature, worked example. If you download the NSW RFS Firefighter Pocketbook from the app store, you can see for yourself that all else being equal (say, 10% relative humidity, 15km/hr wind speed, lowest fuel load 1, drought factor 10, slope 0) an increase from 30 degrees to 45 degrees changes the FFDI from 34 (Very High) to 57 (Severe).
However, under extreme and catastrophic fire conditions, the McArthur model underpredicts the rate of spread and intensity. When you exceed a FFDI of around 50 you switch from a fuel-dominated to a weather-dominated fire. This is why there is a lot of focus/media reporting on the weather (air temperature, relative humidity and wind speed) in extreme and catastrophic fire danger rating periods. Basically, in those conditions (like we have been seeing) you only need the tiniest amount of fuel to produce a fire intensity that cannot be fought.
This is also why, with bushfires associated with an FFDI of >100, even in areas that have had hazard reduction burns less than five years prior, there has been proved to be no measurable effect on the intensity of the fires.
Prescribed burns, aka hazard-reduction fires, is therefore useful, necessary, and effective for lesser conditions, but even if you were to hazard burn everything to the max, its not going to give a benefit under ‘catastrophic’ fire weather conditions. Also, its not possible to hazard burn some of the areas now burning in NSW/VIC in cooler months, it is too wet.
But in anycase, the national parks have been hazard burnt. In the last 8 years in NSW, the NSW Parks & Wildlife carried out hazard reduction burns over 680,000ha – more than double that of the previous 5-year period. In QLD, in 2019 QLD Parks hazard burned 1.4M ha the largest area covered in the last 6 years. So there has been no ‘locking up’ of the national parks, they have been hazard burning more than ever, and yet the fires still burn – because of the weather, which of course, has been made worse by climate change.
There has been 3 years of drought, and 2019 was the lowest rainfall on record in many parts of Australia.
The humidity in the parts of NSW and Victoria on fire now has been very low <10%, and there has been strong winds.
Then, it has been very very hot, temperature records have fallen, 2019 hottest on record, and also hot at night.
All those together mean when the fires go, they burn like nothing seen before.