The red-hot start to   October barely let up, setting Australia up for its most abnormally warm month in records going back to 1910, the Bureau of Meteorology said.
The arrival of summer-like conditions several months early has combined with on-going dry conditions in many southern parts of the country to create dangerous fire conditions, authorities say.
Nationally, maximum temperatures were 3.44 degrees above average for   October, eclipsing the previous record deviation of 3.41 degrees set in   September 2013, the bureau said its monthly climate report. 
The unusual heat was even more notable for mean temperatures, which take the average of days and nights. Last month was 2.89 degrees warmer than the long-term average, beating the previous record mean anomaly of 2.75 degrees, also set in   September 2013. Minimum temperatures, too, set a new high for the month.
Almost the entire southern half of the country posted record heat during   October:
"The extreme monthly anomalies were a result of exceptional early-season warmth at the start of the month and persistence of above-average temperatures throughout the month as a whole," the bureau said.
Compared with the previous warmest   October in 1988, last month was 0.7 degrees warmer.
"Certainly it's an   October that was hotter than most   Novembers," Karl Braganza, head of climate monitoring at the bureau. The national mean temperature came in at 25.6 degrees.
The large El Nino event in the Pacific is one influence driving the warmth, as are conditions in the Indian Ocean that have favoured a reduction in rainfall. With less rain, evaporative cooling is reduced, allowing temperatures to rise further.
Last month, rainfall was 53 per cent below average nationally, extending the dry spell for many parts of eastern Australia, especially in Victoria and Tasmania.
The combination of summer-like temperatures and dry conditions is expected to prompt fire authorities to update their seasonal outlooks, with more regions likely to face a busier-than-usual bushfire season.
  September was Australia's third driest on record and last month extended the dry trend.
"  September and   October will have a significant impact on the subsequent fire season," Dr Braganza said.
The coming weeks could see some good rainfall recorded across Australia but the falls would need to be substantial to make up for low soil moisture levels in many regions, he said.
NSW, Victorian scorchers
Among the states, New South Wales, Victoria, South Australia, and Western Australia all observed their warmest   October on record, the bureau said. 
In NSW, average temperatures were 4.52 degrees above the norm for   October, beating the previous record set just a year earlier by a huge 1.94 degrees. Maximum temperatures were 5.50 degrees above normal. Statewide, rainfall was 49 per cent of the usual for the month.
Sydney had its warmest nights on record for any   October, while daytime temperatures were the fifth warmest. Overall it was the city's second-warmest   October, trailing only 1998.
Melbourne posted its hottest   October, as did Canberra, Adelaide and Perth, while Hobart matched the mark reached in 1963, Weatherzone said.
For Victoria alone, both average maximum and mean temperatures beat the previous record by about 1 degree. Mean temperatures were 4.13 degrees above the average for   October, easily eclipsing the 2.99 anomaly set back in 1914, the bureau said. Maximum temperatures were also 5.93 degrees above average compared with the previous record anomaly for   October of 4.98 degrees, also set in 1914.
For the month, rainfall was just over one quarter of the long-term average for the month, making it Victoria's seventh-driest   October on record, the bureau said.
Global warmth
The record hot   October in Australia may be a pointer to another month of abnormally warm global temperatures.
The El Nino event, which is marked by the Pacific Ocean absorbing less heat than usual from the atmosphere, helped drive worldwide temperatures in   September to the most anomalously hot month in 1629 months tracked by US agencies going back to 1880.
El Ninos typically add 0.1-0.2 degrees to global temperatures, a boost that is adding to the background warming from climate change, scientists say.
During El Ninos, the usual easterly trade winds stall or are reversed, resulting in a build-up of heat in the central and eastern equatorial Pacific. Rainfall patterns also tend to shift eastwards away from the western Pacific, bringing drought to areas such as Papua New Guinea and Indonesia - as is now being experienced in those countries.

Ten most anomalously warm months
  October 2015      2.89 degrees (above mean-temperature average)
   September 2013  2.75 
   April 2005             2.66
   August 2009         2.47
   June 1996            2.37
   October 1988       2.14
   June 1957            2.12
   May 1958             2.11
   July 1973             1.96
   June 1991            1.94
Source: Bureau of Meteorology