what’s behind the intense, sudden fires burning across New South Wales and Tasmania
- Written by: Rachael Helene Nolan, Associate Professor, Hawkesbury Institute for the Environment, Western Sydney University
Dozens of bushfires raged over the weekend as far afield as the mid-north coast of New South Wales and Tasmania’s east coast. A NSW firefighter tragically lost his life, 16 homes burned down in the NSW town of Koolewong and four in Bulahdelah, and another 19 burned down in Tasmania’s Dolphin Sands.
Temperatures reached 41°C in Koolewong. Strong winds fanned the fires, making them hard to suppress. The speed and intensity of these fires took many by surprise. Why did they do such damage?
Since the megafires of the 2019–20 summer, Australia has had multiple wet years. Vegetation has regrown strongly. In recent months, below-average rainfall has dried out many landscapes, resulting in dry fuels ready to burn. Prime Minister Anthony Albanese has warned these fires point to a “difficult” season ahead.
Do these fires mean Australia is facing another terrible fire season? Not necessarily. The growth of fuel is one thing. But a lot depends on the weather.
Dry fuel, steep terrain and poor access
Big, dangerous bushfires need fuel to burn. Scientists categorise these fuels as either “dead” or “live”. Dead fuels include fallen leaves and twigs on the ground, whereas live fuels include the foliage on living shrubs and trees.
When fuel is wet, it doesn’t burn easily. But when it’s very dry, it burns readily. Drying can happen very quickly on days of high temperatures and low humidity. Fuel dryness can be estimated from weather or satellite data.
Our work has shown fires can spread much more easily when moisture levels drop below 10%.
Over the weekend, we calculated the moisture content of dead fuels fell to critically dry levels, falling below 7% in both Koolewong and Bulahdelah on Saturday December 6. These estimates of dead fuel moisture are derived from our model, which calculates moisture content from gridded maps of temperature and humidity.
Low fuel moisture and strong winds mean higher fire risk. But there are other factors too.
The majority of homes lost in NSW were in Koolewong, just south of Gosford on the Central Coast. Steep terrain and poor access may have contributed to these losses.
Many homes in this region have been built close to eucalypt forests. We know houses are more likely to be destroyed by fire if situated in areas where forests make up more than 60% of the surrounding neighbourhood, compared with houses with a low percentage of surrounding forest.
Many homes here would have been built before current building codes that require bushfire-resistant construction came into effect.
Because performing hazard-reduction burns around homes in forest landscapes is challenging, there’s a greater onus on homeowners in forested areas to ensure their homes are prepared for fire as best as possible. Sometimes, even this won’t be enough.
Primed for fire once again
The fires at Koolewong and Bulahdelah burned through forests that narrowly escaped the 2019–20 Black Summer fires. These megafires burned more than 7.2 million hectares of southern Australia, an area larger than European nations such as Ireland and Denmark. NSW was hardest hit.
Since that devastating summer, NSW has had a reprieve. Years of wetter-than-average conditions followed, with the exception of 2023.
Bushfires burn through the built-up fuel on the ground, making fires in following years less likely. This protective effect is usually strongest in the first five years after fire.
This summer marks six years since the Black Summer. Wet conditions have meant fuel loads are fast recovering – especially in the tracts of land where severe fires raged, burning up into the canopy. After the fires, the blackened landscape was left with high light levels. This, coupled with several very wet years, has led to ideal conditions for vegetation growth.
Our recent research shows there are now very high levels of midstorey fuels in many areas – flammable shrubs and regenerating eucalypts. These elevated fuels can make a fire much more intense. That’s because they act as a ladder for flames burning along the forest floor to reach up into the trees and potentially start a canopy fire.
What lies ahead?
Drought conditions have now eased for much of southern Australia – with the exception of eastern NSW.
The Bureau of Meteorology has declared a weak La Niña event is in progress. These events typically bring wetter, cooler conditions to much of Australia. But this one seems weak, and climate change is making it harder to predict them based on the historic record. Long-range forecasts predict rainfall is actually likely to be lower than average over December.
These intense fires and dry conditions mean we should be careful this fire season – especially in drought-affected eastern NSW.
Authors: Rachael Helene Nolan, Associate Professor, Hawkesbury Institute for the Environment, Western Sydney University
