A huge swath of central and south-eastern Australia is poised to swelter on Friday with temperatures expected to peak at 37 degrees in Canberra, 41 degrees in Melbourne, 42 degrees in Adelaide and even 38 degrees in Hobart.

If the ACT reaches the forecast top of 39 degrees on Saturday it will be the hottest day in three years. If it reaches 40 degrees, it will be the first time Canberra's mercury has risen that high since February 2009. But Canberra will be spared the full force of the blazing heat, with maximums slightly tempered by very light easterly breezes keeping the hot air from the north-west at bay, according to Bureau of Meteorology duty forecaster Magda Galos-Lorenc.

''Definitely it will be hot. The temperature for the whole week will be above average,'' Ms Galos-Lorenc said. ''We will have easterly onshore flow, which will bring cooler temperature from the ocean, and also a little bit more cloud.''

It was these breezes that quickly cooled the capital on Wednesday evening, sending the temperature down to a below-average minimum of 12 degrees overnight.

''It's quite common for Canberra to get this easterly wind change. We call it the easterly surge … the temperatures drop significantly and quite quickly.'' But Ms Galos-Lorenc said the same cooling breeze would not be felt as drastically on Friday evening, with the warmth of a 37-degree day expected to linger into the night.

After forecast tops of 37 and 39 degrees over the next two days, a weak change will bring some minor relief to the capital.

Nationwide, the record to be challenged is the 40.17 degrees average

maximum reached on December 21, 1972. The country notched up an average maximum of 39.21 degrees on Wednesday, as measured across more than 700 weather sites. That result was narrowly outside the top 10 days recorded since 1950, the NSW climate services manager at the bureau, Aaron Coutts-Smith, said.

The new record could be reached on the weekend or even Monday, so prolonged is the present heat spell.

Also within range could be the highest temperature ever recorded in Australia, of 50.7 degrees at Oodnadatta in South Australia on January 2, 1960. Several sites in WA reached 47.9 degrees on Thursday.

Australia has been warming up for some months after the start of last year was unusually cool at the tail-end of a La Nina climate pattern that brought wet summer conditions for the last two summers.

The final four months of the year saw the hottest average maximum temperatures ever recorded for that period, the bureau said on Thursday as it released its annual Australian Climate Statement 2012. Canberra's 2012 was the first cooler-than-average year since 1996, with the capital shivering through its coldest nights since 1972.

There were 88 nights last year where the temperature bottomed out at 0 degrees or colder, 28 nights more than average.

It was the driest December since 2006 in Canberra, with just 37 millimetres recorded at the airport, about 17 millimetres below average. And despite above-average maximum temperatures, Canberra also had its coldest December night ever, when the mercury dropped to 0.6 degrees on December 6, and the coldest Christmas day in more than 30 years.

Nationally, average maximum temperatures for the September-December period were 1.61 degrees above the 1961-90 average, at 32.47 degrees, narrowly breaking the previous record for the period set in 2002.

The overall figures for 2012 show the average temperature for the year was 0.11 degrees above the 1961-90 mean of 21.81 degrees. Rainfall averaged 476 millimetres, compared with a 465-millimetre mean for the 1961-1990 period - well down from the 699 millimetres recorded for 2011.

Within the year, though, Australia saw a major shift in conditions as both hotter and drier conditions set in. For instance, while the January-March rainfall levels were 32 per cent above average, they slumped to be 25 per cent below the norm for the April-December period.

''Are we going back to sort of average years after a La Nina event or are we going back to those dry conditions that really characterised the southern Murray-Darling Basin?'' Karl Braganza, manager of climate monitoring at the Bureau of Meteorology, said. ''Based on the evidence we've seen, we are going back to drier-than-average conditions.

Last month the British Meteorological Office predicted that this year would be among the hottest for the globe on record, with increased emissions of greenhouse gases a key factor.

Australian forecasters are reluctant to estimate how this year will turn out for the country mostly because of uncertainty in the balance between La Nina and El Nino climate patterns over the Pacific Ocean. El Nino conditions typically mean hotter and drier conditions for much of the country.

''Two thousand and nine was a really warm year, and that's the last time we had these really warm air masses moving over the continent from quite early on [in the summer]'', Dr Braganza said.

Even after the present heatwave subsides, Australians shouldn't expect much respite from a long, hot summer. ''Odds are in favour of hotter and drier conditions to the end of March,'' Dr Braganza said.