Australia's Great Western Woodlands are the largest intact temperate woodland ecosystem on Earth, spanning an area the size of the average European country. These woodlands are part of one of the world's biodiversity hotspots and, despite subsisting on just 200–400 mm of rainfall a year, can store considerable amounts of carbon. However, they face growing pressure from a combination of climate change and increasingly frequent and large wildfires, which have burned over a third of these slow-growing, fire-sensitive woodlands in last 50 years alone. To develop conservation strategies that bolster the long-term resilience of this unique ecosystem, we urgently need to understand how much old-growth woodland habitat remains intact and where it is distributed across this vast region. To tackle this challenge, we brought together data from an extensive network of field plots distributed across the region and combined this with information on vegetation 3D structure derived from drone, airborne and spaceborne LiDAR. Using this unique dataset, we developed a novel modelling framework to generate the first high-resolution maps of woodland tree size and age structure across the entire region. We found that 41.2% of the woodland habitat is covered by old-growth stands, equivalent to an area of approximately 39,187 km2. Only 10% of these old-growth woodlands fall within current protected areas managed by the state government. Instead, most remaining old-growth woodlands are found either within the Ngadju Indigenous Protected Area (26.9%) or outside of formal protected areas on leaseholds and privately owned lands (57.2%). Our maps of woodland size and age structure will help guide the targeted management and conservation of the Great Western Woodlands. Moreover, by developing a robust pipeline for integrating LiDAR data from multiple platforms, our study paves the way for mapping the 3D structure and carbon storage of open and heterogeneous woodland ecosystems from space. •Novel framework fusing drone, airborne and satellite LiDAR for large-scale mapping.•Fires have burned 39% of the world’s largest temperate woodland in just 50 years.•Old-growth woodlands still cover 41% the region, equivalent to 38,715 km2.•Only 10% of old-growth woodlands are in protected areas managed by the government.