On 4 August 1986, torrential rainfall occurred in Sydney, with 327.6 mm recorded in a 24 hour period. In the normally drier western suburbs, rainfall levels of 250mm were recorded.

Damaging floods occurred as a result. Around 10,000 homes were affected, and transport severely disrupted: flooded roads stranded many people in the city, while bus and ferry services were also affected.

Electricity supply problems were reported as well as other property damage. As the rain eased in Sydney the wind shifted in a southerly direction early on 6 August. This caused the Blue Mountains to experience heavy rains, resulting in the largest floods on record on the Cox and Grose rivers. The rain moved south to the Illawarra and west of Wollongong, adding to the already high Georges River (recorded at 4.5 m), while; the Hawkesbury River peaked at 12.8 m. This resulted in flooding covering the bridge at Milperra. Six deaths were recorded.

The Insurance Council of Australia estimated the 1986 damage at $35 million, with the 2011 estimated normalised cost of $215 million.

As the rain eased in Sydney the wind shifted more southerly early on 6 August. This caused the Blue Mountains to experience heavy rains, resulting in the largest floods on record on the Cox and Grose rivers. The rain moved south to the Illawarra and west of Wollongong, adding to the already high Georges River (recorded at 4.5 m); the Hawkesbury River peaked at 12.8 m. This resulted in flooding covering the bridge at Milperra. Six deaths were recorded.

The Insurance Council of Australia estimated the 1986 damage at $35 million, with the 2011 estimated normalised cost of $215 million.