The extreme heat prompted the Bureau of Meteorology to issue a Special Climate Statement, confirming record temperatures across NSW, South Australia, Tasmania and Victoria, where the highest daily minimum temperature ever recorded was reached (31.9 degrees in Mildura).

"The most intense phase of the heatwave began on December 16 as high pressure became established in the Tasman Sea and directed hot, north-easterly winds over South Australia," the bureau said.

"The heat spread over much of south-eastern Australia from 18 December as winds turned more northerly, reaching its most intense levels over the weekend of 19-20 December. A trough and cold front crossed the region on 20 December, bringing the heatwave to an end over the most-affected areas although hot conditions continued over parts of New South Wales on the 21st."

Sydneysiders have surely not forgotten the night of 20th, when they sweated through the hottest December night in 15 years, during which the mercury was still sitting at 29 degrees at 10pm in the city, before dropping briefly to a low of 22.6 degrees just after 3am.

An extended period of hot weather in South Australia concentrated on Adelaide, where temperatures reached 40 degrees on each of the four days from December 16 to 19.

"This was the first occasion that four consecutive days of 40 degrees or above had occurred in Adelaide in December," the bureau said.

"The highest temperatures of the heatwave occurred on 19 December. Hottest of all was the upper Spencer Gulf region, where Port Augusta reached 47.2 degrees, with 45.8 degrees at Whyalla and 45.6 degrees at Port Pirie."

Bureau senior climatologist Blair Trewin said the South Australian heatwave was particularly interesting as heatwaves usually occurred in late summer.

"Systems tend to be more stable and slow moving," he said. "It's unusual to get a heatwave in December. We've had that a few times in January and February but never December."

However, the fact a heatwave occurred early in summer did not suggest even hotter conditions for the coming January and February, Mr Trewin said.

"The seasonal climate outlook is leaning towards cooler conditions in much of Victoria and South Australia," he said. "We are experiencing a strong El Nino, but the main effect of that on temperatures in Southern Australia is actually in the second half of the year.

"El Nino effects on average temperatures disappear in Southern Australia from January onwards."

In Victoria, El Nino summers tend to bring more extremes at both ends of the scale, meaning more hot days but also more unusually cool temperatures as well.

The remarkable global heat experienced this year may not be the last of it, with forecasters already predicting next year will be hotter again – marking three years in a row of record annual warmth.

The prediction, by Britain's Met Office, came just days after almost 200 nations agreed in Paris to a new global agreement to tackle climate change.

Under the pact, to take effect from 2020, nations would review efforts to curb greenhouse gas emissions every five years with the aim of keeping temperature increases to "well below 2 degrees" of pre-industrial levels.