Environmental context In 2009, at the end of the longest drought period ever recorded in Australia, a major dust storm blanketed the cities of Sydney and Brisbane for more than 24 h. The source of the dust was inner New South Wales and South Australia, where large scale open-cut mining occurs together with agricultural practices. We report results of extensive mineralogical and chemical analyses of the dust, and discuss their significance in terms of the dust origins and potential human health risks. Abstract In a 24-h period from 23 to 24 September 2009, a dust storm passed over Sydney, Australia that produced a red sky and reduced the visibility to a few metres. It was Sydney’s worst dust storm since 1942. During this period, the PM10 (particles measuring 10 μm or less) value jumped from 50 to 11 800 µg m–3. The dust storm was sampled and its mineralogical and organic contents were analysed. Four major particle sizes (0.6, 4.5, 9.3 and 20 µm) were observed in the dust. A multimodal particle distribution indicated a long range of dust transport. Mineralogical analysis showed that the particles were mainly composed of crustal elemental oxides of Al and Si. The ratio of Al/Si was 0.39 and the organic content was 10.6 %, which was found to be enriched with humic-type substances. The high Al/Si ratio (>0.3) indicated that the dust originated from desert land whereas the high organic content indicated that the particles were also derived from eroded agricultural land. A fluorescence spectroscopic study on the organic matter at excitation and emission wavelengths of 245–265 and 330–350 nm indicated that biohazardous substances were unlikely to be present in the dust.