At midday AEST on Monday, the benchmark S&P/ASX200 index was down 65.8 points, or 0.8 per cent, to 8,144.0, while the broader All Ordinaries was down 65.5 points, or 0.78 per cent, to 8,371.7.
Moomoo market strategist Jessica Amir attributed part of the drop to falling commodity prices given recent economic weakness in China.
"Miners will have a grim day as iron ore prices are down 3.4 per cent today, and aluminium is 2.2 per cent lower," Ms Amir said.
The Reserve Bank is widely expected to leave rates on hold Tuesday, which JP Morgan analysts noted on Monday would leave it "quite out of step with the global mood music".
Eight of the ASX's 11 sectors were lower at midday, with energy, tech and utilities all up by roughly 0.4 per cent.
The consumer staples sector was the biggest loser, dropping 2.8 per cent after the Australian Competition and Consumer Commission launched legal action against Woolworths and Coles for allegedly misleading buyers about their discounts.
Woolworths shares were down 3.2 per cent to a six-week low of $33.87, while Coles had dropped 3.4 per cent to a two-week low of $18.565.
Elsewhere in the sector, bottle shop owner Endeavour Group was down 2.2 per cent and Treasury Wine Estate fell 2.8 per cent, while in the discretionary sector, Kmart owner Wesfarmers had dipped 2.0 per cent.
In the energy sector, a trio of uranium developers were the three biggest gainers at midday after it was announced that the shuttered Three Mile Island nuclear plant in the US would be reactivated to provide energy for Microsoft's artificial intelligence operations.
Boss Energy was up 7.3 per cent, Deep Yellow climbed 6.0 per cent, and Paladin Energy rose 6.5 per cent.
Helius was up 3.4 per cent after the pathology chain announced plans to sell its diagnostic imaging business to Hong Kong-based private equity firm Affinity Equity Partners for $965 million.
"The sale with provide Healius with both the resources and time to continue to improve our pathology operations and the scope to return cash to shareholders," Helius chief executive Paul Anderson said.
In the heavyweight mining sector, BHP was down 1.3 per cent and Rio Tinto slid 0.6 per cent, while Fortescue added 0.2 per cent.
The big four banks were all in the red, with ANZ down 0.8 per cent, NAB dipping 0.4 per cent, Westpac and CBA slipped 0.2 per cent and 1.2 per cent respectively.
In the tech sector, Appen was up 13.1 per cent to a one-year high of $1.99, its shares having nearly doubled so far this month after reaching a nadir of 27c in January.
In the consumer discretionary sector, the company formerly known as Webjet Limited completed the spinoff of its namesake business-to-consumer travel business. That company is now known as WEB Travel Group, a business-to-business platform serving the hotel industry while the new spinoff has been named Webjet Group.
WEB's shares were down 10.8 per cent to $7.34 while Webjet Group's shares had begun trading at 79.75c.
The Australian dollar was buying 68.16 US cents, from 68.20 US cents at Friday's ASX close.
22°C