Nationals senator Matt Canavan told an inquiry on Wednesday it was a "huge gap and scandal" that the market body could not model whether power prices would be lower in five years time as renewable energy takes over.
The latest system plan sets out the new generation, energy storage and transmission needed to achieve net-zero by 2050, at a cost of $122 billion - not including the costs of building new poles and wires.
Giving evidence to the Senate inquiry into energy planning and regulation, AEMO's chief executive Daniel Westerman said he "can't guarantee" power bills will fall.
Daniel Westerman says it's impossible to give a guarantee that power bills will fall. (Dean Lewins/AAP PHOTOS)
"We work to the national electricity objectives designed to promote the efficient investment and operation of energy services in the long-term interests of consumers," he said.
The plan for the development of the power system is required to deliver reliability and security and reflects government policy settings, including emissions reduction targets.
Executive general manage for system design Merryn York said they follow a "least-cost" pathway under the market-based system, which "should deliver the lowest price".
Merryn York says the energy market operator is following a "least-cost" pathway. (Mick Tsikas/AAP PHOTOS)
In April, the nation's energy ministers agreed to expand the approach to include demand forecasting for consumer energy resources such as rooftop solar and electric vehicles, as well as the interactions between electricity and gas.
"The pathway that Australia is on is a very common one," Mr Westerman said.
More recently, governments have requested AEMO take on new functions to further support the energy transition, he said, including the capacity investment scheme designed to boost renewable sources by 2030.
But energy giants have called for reforms to how the national electricity market operates beyond 2030, by which time it should be operating with 82 per cent renewables under existing targets.
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