A moderate-strength downslope windstorm occurred in the Sydney Metropolitan Area during the early morning of 31 October 2023. It was a rare event in view of the climatology of Sydney, as some stations reported the highest wind gusts in the last 20 years. The event was initiated by a rapidly propagating disturbance in a vertically sheared wind environment ahead of the passage of a trough. With the assumption of a quasi-steady condition, the phase speed of the disturbance translated to a reduction of the effective cross-barrier winds, which resulted in the wave breaking in the low to mid-troposphere. The flow transition from sub- (F < 1) to supercritical (F > 1) across a low mountain barrier ~900 m high was possible owing to the alignment of several factors. These include the right tuning of the phase speed relative to the ambient westerly winds, the split flows and wave breaking at the right altitude, and the linear wind shear upstream, which caused a hydraulic jump response, and the downslope acceleration on the lee side of the Great Dividing Range. The close match of the barrier height of the Oberon Plateau to the critical ridgetop height was remarkable, suggesting that the wind speed observed during the event was likely the maximum attainable wind speed. Further analysis shows that a classical Boulder-style downslope windstorm involving a deep tropospheric layer and forcing by a slow-moving synoptic system is unlikely in Sydney.