Melbourne has a moderate oceanic climate (Köppen climate classification Cfb) and is well known for its changeable weather conditions. This is mainly due to Melbourne's location situated on the boundary of the very hot inland areas and the cold southern ocean. This temperature differential is most pronounced in the spring and summer months and can cause very strong cold fronts to form. These cold fronts can be responsible for all sorts of severe weather from gales to severe thunderstorms and hail, large temperature drops, and heavy rain. Port Phillip is often warmer than the surrounding oceans and/or the land mass, particularly in spring and autumn; this can set up a kind of "bay effect" similar to the "lake effect" seen in the United States where showers are intensified leeward of the bay. Relatively narrow streams of heavy showers can often affect the same places (usually the eastern suburbs) for an extended period of time, whilst the rest of Melbourne and surrounds stays dry. Melbourne is also prone to isolated convective showers forming when a cold pool crosses the state, especially if there is considerable daytime heating. These showers are often heavy and can contain hail and squalls and significant drops in temperature, but they pass through very quickly at times with a rapid clearing trend to sunny and relatively calm weather and the temperature rising back to what it was before the shower. This occurs often in the space of minutes and can be repeated many times in a day, giving Melbourne a reputation for having "four seasons in one day", a phrase that is part of local popular culture and familiar to many visitors to the city.

Melbourne is colder than other mainland Australian state capital cities in the winter. The lowest temperature on record is ?2.8 °C (27 °F), on 4 July 1901. However, snowfalls are rare: the most recent occurrence of sleet in the CBD was on 25 July 1986 and the most recent snowfalls in the outer eastern suburbs and Mount Dandenong were on 10 August 2005, More commonly, Melbourne experiences frosts and fog in winter.

During the spring, Melbourne commonly enjoys extended periods of mild weather and clear skies. Melbourne and Sydney's average January and February daily highs are similar. However, Melbourne's summers are notable for days of extreme heat, with Melbourne holding the Australian capital city extreme temperature record of 46.4°C, set on 7 February 2009.