At noon AEST on Wednesday, the benchmark S&P/ASX200 index was down 23 points, or 0.29 per cent, to 7,988.9, while the broader All Ordinaries was down 18.1 points, or 0.22 per cent, to 8,198.4.
Overnight Brent crude tumbled nearly four per cent to trade under $US69 a barrel for the first time since December 2021 after the Opec+ oil cartel cut its forecast for oil demand in 2024 and 2025.
Traders were also watching as Kamala Harris and Donald Trump squared off for what's scheduled to be their only US presidential debate ahead of the November election.
Eight of the ASX's 11 sectors were lower at midday, with mining, utilities and property higher.
Tech was the biggest loser, dropping 1.5 per cent. NextDC had slid 4.8 per cent after the data centre owner completed a $550 million placement, while Xero was down 1.2 per cent and Life360 had fallen 1.8 per cent.
All of the big four banks were down, with ANZ dropping 1.4 per cent, Westpac dipping 1.5 per cent, NAB retreating 1.2 per cent and CBA subtracting 1.9 per cent.
In the energy sector, Woodside was down 1.9 per cent and Santos had drooped 1.2 per cent amid the plunging oil price.
The heavyweight mining sector was up 1.5 per cent, with BHP adding 1.3 per cent, Fortescue climbing 2.6 per cent and Rio Tinto advancing 0.8 per cent.
Goldminers were also higher as the precious metal changed hands at a near all-time high of $US2,519 an ounce. Northern Star had advanced 1.7 per cent, Evolution had grown 2.2 per cent and Newmont had added 0.7 per cent.
Lithium miners had posted double-digit gains following months of decline, after a UBS analyst said CATL had halted lithium mining at two of its mines in southeast China.
Mineral Resources had soared 17.2 per cent to $35.50, Pilbara Minerals advanced 15.7 per cent and Liontown has grown 17.2 per cent.
In small caps, Melbana Energy had soared 63 per cent to a two-month high of 3.1c after the Sydney-headquartered company announced work was progressing on its oil well in Cuba.
The Australian dollar was buying 66.58 US cents, from 66.73 US cents at Tuesday's ASX close.
