Map from Google - Sydney

The climate of Sydney, capital of New South Wales, and lying on the southeast coast of Australia, is subtropical oceanic, with mild winters and warm summers.
Here are the average temperatures.
Average temperatures - Sydney
SydneyJanFebMarAprMayJunJulAugSepOctNovDec
SydneyJFMAMJJASOND
Min (°C)181817141198911131617
Max (°C)262624221916161719222325
Min (°F)646463575248464852556163
Max (°F)797975726661616366727377

Rainfall is fairly abundant, since it is close to 1,200 millimetres (47 inches) per year. There is no dry season, however, the rainiest season is autumn.
Although the climate of Sydney is generally good, sometimes there may be particularly intense rains, thunderstorms and hailstorms.
Here is the average precipitation.
Average precipitation - Sydney
SydneyJanFebMarAprMayJunJulAugSepOctNovDecYear
SydneyJFMAMJJASONDY
Prec. (mm)9010013013513012012075757075751195
Prec. (in)3.53.95.15.35.14.74.7332.83347
Days109101098888889105

The wind blows frequently, especially from October to April, that is, in the warm season (being in the Southern Hemisphere, the seasons in Sydney are reversed to those of Europe or North America).
In the warm half-year, from October to March, heatwaves may occur, with the wind blowing from the desert, which can raise the temperature above 35 °C (95 °F), though for short periods (sometimes even one day); in addition, the wind can bring sand and dust from the desert.

Winter, from June to August, is mild, given that the average temperature of the coldest month, July, is 12 °C (53.5 °F). In practice, it's never very cold nor it freezes, even though slight frosts can be recorded occasionally in the western suburbs, more distant from the sea. However, winter is a rainy season, especially in the first part (June).

Spring, from September to November, is mild or pleasantly warm, sunny enough and generally not too rainy.

Summer, from December to March, is warm, with average temperatures not too high, and generally pleasant: highs are on average about 25/26 °C (77/79 °F). Sea breezes blow constantly in the afternoon, and sometimes can be intense.
The sun shines on a regular basis, even though some cloudiness can form quite often, associated with showers and thunderstorms, which typically do not last long. However, every so often the remnants of a tropical cyclone can get this far, bringing heavy rains and strong winds, especially from January to March.
Although the heat is generally not excessive, the wind from the desert can raise the temperature in a spectacular way, though for short periods. In January 2013 the temperature reached 46 °C (115 °F), though for just one day. The wind from the desert is strong and dry, and sometimes can cause fires in the hills behind the town, so that the air can be filled with smoke.

Autumn, in April and May, is very mild, but it is also a rainy season. During some years the rains can be plentiful.

The amount of sunshine in Sydney is good throughout the year; the sun can shine in winter as well. However, there is no season in which the weather is always good.

Bondi Beach

For swimming, in Bondi Beach the sea is warm enough from January to March, when the ocean temperature is about 24 °C (75 °F), while it drops to 19 °C (66 °F) from July to October (therefore it becomes a bit cold). On the ocean coast, people surf, while in the interior of the bays and in Botany Bay the sea is a bit warmer and calmer.
Here are the average sea temperatures.
Sea temperature - Sydney
SydneyJanFebMarAprMayJunJulAugSepOctNovDec
SydneyJFMAMJJASOND
Sea (°C)242424232120191919192122
Sea (°F)757575737068666666667072

When to go

The best season to go to Sydney is spring, in September and October, since it is mild, fairly dry and sunny. In November the rains increase a bit, but not like in summer, and heat waves are still very rare, so November is a good month as well. On the other hand, there is no period to be avoided: it depends greatly on year and period. Anyway, autumn is rainy, while in summer, which is typically a good season, torrid days can sometimes occur, as well as some intense thunderstorms, and some very rainy periods.