Even if the Paris agreement to limit the global temperature rise to below 2C is met, summer heatwaves in major Australian cities are likely to reach highs of 50C by 2040, a study published on Wednesday warns.

Researchers led by the Australian National University in Canberra used observational data and simulated climate models to assess future extreme weather events in New South Wales and Victoria. They examined what these weather extremes might look like even if the Paris agreement target of limiting climate change to a 2C increase is met.

The lead author of the study, the climate scientist Dr Sophie Lewis, said Sydney and Melbourne could expect unprecedented summer temperatures of 50C under two degrees of global warming.

The researchers concluded this could be avoided by limiting global warming to 1.5C, the best-case scenario target set under the Paris agreement, though the researchers did not project what temperatures could be expected under this reduced global target.

“What previous studies have done is look at changes in the frequency of record-breaking temperatures so, for example, how much more common 40-degree days might be in future,” Lewis said.

“Our study wanted to look at what the maximum temperatures in an extreme summer of the future would be. That is what we need to know to plan for the future. We know that two degrees of global warming doesn’t sound like much of an increase but it in fact will lead to extreme weather events becoming more severe.”

Governments needed to start thinking about how the public transport system would cope during peak hour in extreme temperatures, how emergency departments would respond to increased demand from elderly people and others vulnerable to heatstroke, and how energy requirements would be met during peak temperatures, she said.

“The only thing we can do to prevent these extremes is to reduce our greenhouse gas emissions as quickly and deeply as we can, though some warming is already locked in the system so we will see some increase in the extremity of temperatures regardless,” Lewis said.

The research was published in the science journal Geophysical Research Letters.

The Environment Victoria chief executive, Mark Wakeham, described the findings as “alarming”.

“It’s preposterous that in the face of these warnings and clear signals that we don’t have a plan to cut emissions,” he told Guardian Australia. “Beyond that we must dramatically improve our built environment to cope.”

Housing standards needed to be improved, including retrofitting older homes to make them more energy efficient and cooler, and more shade needed to be incorporated into cities and towns, he said.

“We also need updated emergency management plans for extreme heatwaves,” he said. “This study by respected researchers confirms that climate change is not just an environmental disaster but it presents an enormous challenge to human health and the economy.

“I don’t think we have any plans in place that would be adequate to withstand days of 50 degrees and it is another urgent warning to our leaders and all levels of government that we need a strong plan to cut commissions and deal with climate change.”