The boss of one of Australia’s largest energy suppliers says she is worried about customers’ power bills after the latest heatwave in the country’s south-east.

Energy Australia’s managing director, Catherine Tanna, has joined the push for a transition to newer forms of energy, saying bipartisanship is needed to draw up a national energy policy. The company operates sites including the Yallourn plant in the Latrobe valley, a brown-coal power station in Victoria that supplies nearly a quarter of the state’s electricity.

“I am worried about our customers and what will happen with their bills,” she told the ABC. “We’ve seen that customers over the weekend in some places in Australia used 25% more than usual.

“In a couple of months when these bills turn up they are going to get a surprise and I am worried about that because I know that the cost of living is a concern for them.”

Tanna warned that over the next 20 years older and cheaper forms of energy – such as coal-fired power stations – were going to retire and she said there needed to be investment in newer technologies such as renewables.

Her warning came after the federal treasurer, Scott Morrison, took a lump of coal into parliament in a stunt before temperature records were broken across Australia. New South Wales had its hottest day on record on Saturday as Penrith recorded 46.9C, while the town of White Cliffs had the hottest night on record at 33.3C.

Bushfires in NSW have destroyed at least 30 homes in the past week while the debate about energy policy reignited. Freedom of information documents this week revealed that the prime minister, Malcolm Turnbull, ignored public service advice when he linked South Australia’s blackouts to renewable energy.

“The single-biggest barrier to investment is uncertainty around policy settings,” Tanna said. “So when there is a lot of rhetoric about policy settings changing, no matter who it comes from, or a lot of flip-flopping about the fiscal assumptions, it makes it very, very difficult for anyone to make a commitment to new projects.”

NSW avoided a blackout on Friday when the demand for electricity reached record highs.

On Monday it was revealed that Turnbull has installed batteries to store energy from his rooftop solar panels so he will have electricity in the event of a power shutdown.

The Daily Telegraph reported that Turnbull had installed lithium-ion batteries at his home in Point Piper and had upgraded his solar panels.