"Some market participants rebid by shifting offers to higher price bands which reduced low-priced capacity further," the Australian Energy Retailer says in a wholesale prices report released on Friday.
August to September had 54 high-price periods, which was the highest number on record for a third quarter and the second highest overall.
Every region recorded at least one high-price period, mostly when wind generation was low and there were network outages, with South Australia accounting for half of them (27).
Big batteries set the price more frequently across the market than a year ago but gas was the dominant price setter.
All regions had high-price periods in the third quarter, often when wind generation was low. (Mick Tsikas/AAP PHOTOS)
Wind generation was more than one fifth (21 per cent) higher compared with a year ago, but variability in output in July and early August led to more use of "expensive firming resources" such as batteries, hydro and fast-start gas plants, the report said.
Prices increased in SA (35 per cent) and Queensland (five per cent) compared with the previous quarter and decreased in NSW (25 per cent), VIC (nine per cent) and TAS (12 per cent).
Year on year prices were significantly higher across all regions, with Tasmania up 290 per cent, Victoria up 114 per cent and South Australia up 76 per cent.
The NEM is a wholesale market where generators and retailers trade electricity, which retailers resell to households and businesses.
"Importantly, forward wholesale electricity prices for 2025 declined slightly this quarter," AER board member Jarrod Ball said.
Forward contract prices were mostly lower in all regions from the December quarter of 2024 through to the end of 2025.
Significant amounts of new clean energy capacity entered the market this quarter as delayed new entrants came online, with approximately 1445 megawatts of capacity added through batteries (600MW), wind (729MW) and solar (116MW).
However, it will take some time before these units reach full output, the regulator said.
Higher winter demand drove higher prices for electricity and gas in July and early August, before the wholesale prices stabilised from mid-August until the end of September amid milder weather.
East coast downstream gas market spot prices decreased by nine per cent to $12.51 per gigajoule from the previous quarter.
Warmer weather also resulted in average gas demand remaining low at 103 petajoules, similar to during mild conditions a year ago when residential and commercial gas demand fell to 10-year lows.
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