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Australia posted its hottest day on record on Monday, with temperatures expected to rise even further on Tuesday.

The average maximum temperature across Australia reached 40.33 degrees on Monday, beating the previous record of 40.17 degrees set in 1972, the Bureau of Meteorology's David Jones told AAP.

Average maximum temperatures have risen above 40 degrees only three times in recorded history.

"We had the hottest day on record for Australia (on Monday) and today it looks like we may well go better again," Dr Jones said.

"This really puts the national dimension of this heat event into bigger context."

Other data from the bureau showed maximum temperatures across the continent in the last four months of 2012 were 1.6 degrees above average, breaking all previous records.

Aaron Coutts-Smith, the bureau's NSW manager for climate services, expects the run of dry and hot conditions to continue for at least the next week.

"What makes this event quite exceptional is how widespread and intense it's been," he said.

"We have been breaking records across all states and territories in Australia over the course of the event so far."

Hobart recorded its hottest day in 120 years on Friday, when the temperature peaked at 41.8 degrees.

Hay, in southwest NSW, climbed to 47.7 degrees on Saturday - its highest in 56 years.

Sydney is expected to reach a maximum of 43 degrees on Tuesday, which would make it the third highest temperature on record.

Markus Donat, from the Climate Change Research Centre at the University of New South Wales, says periods of high temperatures have increased in recent decades.

"In recent studies we have analysed how extreme temperatures have changed globally," he said.

"For most regions, including Australia, we found that extremely high temperatures have become more frequent and more intense."

Dr Donat's research also shows extremely low temperatures have occurred less frequently than they did in the middle of the 20th century.

"Counting the number of very warm days (defined as the warmest five per cent between 1951 and 1980) we found that during the most recent three decades ... the frequency of days in this warmest category has increased by 40 per cent globally," he said.