AEMO analysis showed that a number of energy market records were broken during the quarter, including a record 11.9 gigawatts of rooftop solar generation.
The boom in rooftop solar generation has helped renewables reach a record 70% of total electricity delivered in just half an hour.
But it also underlines the growing pressure on coal-fired power stations, which continue to supply Australia with about 60% of its electricity a year on average, but whose prices are increasingly falling due to cheaper wind and solar power on the global market.
Often, an abundance of solar energy during the day drives wholesale prices to zero or below, forcing coal-fired power plants to run at a loss unless they can reduce capacity.
These conditions help explain the deals the Victorian and New South Wales governments are making with AGL Energy, Energy Australia and Origin Energy to ensure coal-fired power plants don't close over the past few years until alternative energy comes along.
AEMO said wind and solar generators often supply their energy to the wholesale market at negative prices, reflecting the value of the renewable energy certificates they generate, which provide income separate from the electricity market.
The 19% rate is more than double the level in both June of this year and last September. The most negative prices were in South Australia, with prices below zero in a quarter of the cases. When prices dipped below zero, they often dipped below $45 per megawatt hour compared to the previous quarter.
"Record output from renewable generation helped reduce average wholesale electricity prices by more than two-thirds, double the frequency of zero or negative wholesale prices (19%) and reduce overall electricity emissions by 11% compared to the previous September quarter," said AEMO. Executive Director General of Reforms Implementation Violet Moshila Dr.
Above-average temperatures across Australia have helped set several market records, reducing demand for grid power, with home solar providing a greater share of demand. So-called "operating demand," a number that calculates demand for power from the grid, hit a new low of 21,270 megawatts in the third quarter.
Total NEM demand also hit a new low of around 500MW, up from a previous low of 11,393MW after midday on Sunday 17 September, AEMO said.
South Australia, Queensland and Victoria saw record low demand for the quarter, with South Australia's lowest demand of just 21MW on a Saturday in mid-September when rooftop solar supplied 98.5% of the state's main electricity demand.
Record numbers continue to fall this month, with AEMO announcing a new peak for renewable energy penetration in the NEM region of 71.3% from 30 minutes to midday on Friday. Over 44% of the total electricity generated during this period was provided by rooftop solar panels.
Wholesale prices have fallen sharply from their 2022 highs, with the average in the Eastern Mediterranean region down 71% from last year's September quarter to just $63 per megawatt hour.
The highest average price in South Africa is $92 MWh, and the lowest is in Tasmania at $29 MWh.
A report from the Australian Energy Regulator last week showed trend data showing electricity demand fell 5% in the September quarter to a record low, while rooftop solar output rose 31%.
