At 12.00pm (AEST), the benchmark S&P/ASX200 index was down 56.8 points, or 1.48 per cent, at 3771.4 points, while the broader All Ordinaries index lost 53.7 points, or 1.4 per cent, to 3772.9 points.
"It's been a disappointing start to the week,'' Austock Securities senior client adviser and strategist Michael Heffernan said.
"The weakness is in the oil and resources sectors ... and the rest of the market has duly followed.''
Mr Heffernan said it was disappointing that the major indices had fallen below 3800 points again, after not long ago almost touching 4000 points.
Resources were the biggest losers of the morning.
BHP Billiton dropped 2.42 per cent to $32.62, while rival Rio Tinto fell 2.42 per cent to $48.40.
The energy sector was also lower at the noon marker.
At 12.04pm, Woodside had lost 2.46 to $40.41, Santos was down 4.18 per cent at $13.53, and Oil Search had fallen 9 cents to $5.30.
The finance sector was struggling.
Commonwealth Bank was shed 24 cents to $36.95, ANZ lost 16 cents to $15.99, NAB was 5 cents lower at $21.80, and Westpac had dropped 24 cents $19.08.
Among the gold miners, Newcrest was down 11 cents at $30.19, Lihir Gold had fallen 4 cents to $2.86 and Newmont was steady at $5.05.
At 12.12pm, the spot price of gold in Sydney was $US929.80 per fine ounce, down $US3.90 on Friday's close of $US933.70.
On the Sydney Futures Exchange, the September share price index contract was 40 points lower at 3741 on a volume of 9808 contracts.
