Hobart experiences its hottest day on record, with the island capital hitting 41.3C at 1.53pm (NZT 3.53pm), beating a previous high of 40.8C set in January 1976.
That record had stood since 1899, when the mercury hit 40.1C.
Records have been kept in Tasmania since 1883.
The Bureau of Meteorology says it isn’t sure what the new record will be because the temperature is unlikely to peak before 4pm.
Hobart’s overnight minimum was a January record - 23.4C.
RECORDS TO FALL NATIONALLY
The Hobart record fell at the start of a week-long period when other long-standing temperature records were expected to fall across Australia.
A huge swath of central and south-eastern Australia is poised to swelter with temperatures expected to peak at 41 degrees in Melbourne and 44 in Adelaide.
"We probably will get close to some of the really significant Australia-wide records," said Aaron Coutts-Smith, the NSW climate services manager at the weather bureau.
"The majority of Australia is suffering from extreme high temperatures."
Among the records 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, Dr Coutts-Smith said.
The new record could be reached on the weekend or even Monday, so prolonged is the heat spell.
"All three days are looking fairly intense with widespread heat," he said.
Also within range could be the highest temperature recorded in Australia - 50.7 degrees at Oodnadatta in South Australia on January 2, 1960.
Several sites in Western Australia reached 47.9 degrees on Thursday. But the hottest place has been Eucla, where it hit 48.1 degrees on Thursday afternoon, its hottest day on record and 22 degrees above the summer average, said Brett Dutschke, a senior meteorologist with Weatherzone.
While several towns in central Australia are tipped to hit 45 degrees or higher during the heatwave, Dr Coutts-Smith said it was not unheard of for these forecasts to be a little bit out.
"We are watching it closely."
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 conditions for the past two summers.
The final four months of last year had the hottest average maximum temperatures recorded for that period, the bureau said on Thursday, as it released its annual Australian Climate Statement.
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 last year show the average temperature was 0.11 degrees above the 1961-90 mean of 21.81 degrees.
Rainfall averaged 476 millimetres, compared with a 465-millimetre mean for 1961-1990, well down on the 699 millimetres recorded for 2011.
Within the year, Australia experienced a major shift in conditions as 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 the 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?'' said Karl Braganza, the manager of climate monitoring at the weather bureau.
''Based on the evidence we've seen, we are going back to drier-than-average conditions.''
Last month the British Meteorological Office predicted 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.
Dr Braganza said: ''2009 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].''
Even after the heatwave subsides, Australians shouldn't expect much respite from a long, hot summer.
Odds are in favour of hotter and drier conditions continuing to the end of March, Dr Braganza said.
- Sydney Morning Herald