A private equity consortium's plan to buy Origin Energy is on life support after its largest shareholder said it would reject a higher bid described as the "best and final" offer.
AustralianSuper said Brookfield and EIG Partners' $16.4 billion offer "remains substantially below our estimate of Origin's long-term value".
"As per our statement from earlier this week, AustralianSuper believes Origin has a highly strategic portfolio of assets to participate in, and benefit from, the energy transition," it said on Thursday.
Earlier, Brookfield and EIG raised their bid for Australia's largest electricity and gas retailer by $1.2 billion, or eight per cent, after AustralianSuper on Tuesday it would vote against the deal at the November 23 scheme meeting.
AustralianSuper owns a 13.67 stake in Origin after acquiring another 1.02 per cent of the energy giant last month.
Brookfield and EIG Partners' new offer is equivalent of $A9.531 a share, paid in US and Australian dollars, up eight per cent from the $A8.81 per share offered previously.
"Our revised offer price is our best and final offer as to price under the scheme and is a compelling opportunity for shareholders to realise the value of their investment," Brookfield Asia Pacific CEO Stewart Upson said.
Mr Upson reiterated that if the $A16.4 billion acquisition was successful, the consortium would invest $20 billion to $30 billion in renewable energy through Origin over the next 10 years.
EIG chief executive Blair Thomas said it was a compelling offer.
"The fact that no competing offer has surfaced in nearly a year, together with the massive premium in our proposal, evidences the fact that we have identified every element of value available," he said in a statement.
Ron Shamgar, the head of Australian equities at TAMIM Asset Management, posted on X on Thursday that he doubted AustralianSuper and other smaller shareholders would accept what he described as "such a small bump".
"The scheme vote will be close call but most likely to fail," he wrote the platform formerly known as Twitter.
At noon AEDT, Origin shares were down 5.2 per cent to $8.595.