Under the Köppen–Geiger classification, Sydney is classified as having a humid subtropical climate (Cfa), featuring "warm, sometimes hot" summers and "generally mild to cool" winters. The city’s weather patterns are significantly shaped by the El Niño–Southern Oscillation, the Indian Ocean Dipole, and the Southern Annular Mode, which contribute to cycles of drought, bushfire, storms, and flooding.

Proximity to the ocean helps moderate temperatures, but the inland western suburbs often experience more extreme weather. At Sydney's primary weather station at Observatory Hill, recorded temperatures have fluctuated from a scorching 45.8 °C (114.4 °F) on January 18, 2013, to a chilly 2.1 °C (35.8 °F) on June 22, 1932. The central business district (CBD) averages about 14.9 days a year with temperatures hitting or exceeding 30 °C (86 °F), while suburbs can see between 35 and 65 such days annually.

Penrith recorded the hottest temperature in the metropolitan area, soaring to 48.9 °C (120.0 °F) on January 4, 2020. The sea temperature averages between 18.5 °C (65.3 °F) in September and 23.7 °C (74.7 °F) in February, providing a pleasant backdrop for beachgoers. Sydney enjoys approximately 7.2 hours of sunshine per day and around 109.5 clear days each year, although frost can occur in Western Sydney during winter mornings.

The city also experiences an urban heat island effect, making certain coastal suburbs more susceptible to extreme heat. During late spring and summer, temperatures above 35 °C (95 °F) are common, but are often mitigated by a southerly buster—strong winds that bring a quick drop in temperature. Sydney occasionally faces dry, westerly foehn winds, particularly in winter and early spring, which can elevate maximum temperatures.

Rainfall in Sydney exhibits moderate to low variability and is generally consistent year-round. However, recent years have seen a shift toward more summer-dominant precipitation patterns. Most rain falls between late spring and autumn, with reduced levels in late winter to early spring. East coast lows can cause significant rainfall in the CBD during autumn and winter, while summer storms are often fueled by black nor'easters and remnants of cyclones.

While snowfall is a rarity—last reported in 1836—graupel, or soft hail, was mistakenly thought to be snow during a July 2008 event in Upper North Shore. Dry conditions have also led to severe dust storms, the most notable occurring in 2009. Overall, Sydney's climate reflects a dynamic interplay of natural forces, making it a unique and diverse environment.