Defence Minister Kevin Andrews has joined the United States and other nations in opposing China's large-scale land reclamation in the South China Sea.
Satellite imagery shows at least one former atoll in the contested Spratly Islands is now home to a Chinese harbour and a runway is also under construction.
Mr Andrews told a security conference in Singapore that Australia was concerned about any military build-up on the artificial islands in the area believed to be home to oil and gas reserves.
He stressed the need for regional cooperation over territorial matters and issued a warning to China about its activities in the region.
"Australia has made clear its opposition to any coercive or unilateral actions to change the status quo in the South or East China Sea," he said.
"This includes any large-scale land reclamation activity by claimants in the South China Sea, and we are particularly concerned at the prospect of militarisation of artificial structures.