Obama will be adding more troops to Australia over the next year, but it doesn't have anything to do with fearing China, he explained in a joint conference with the Australian Prime Minister. The president said, "We welcome a rising, peaceful China," but the plan will place more Marines at bases located in Australia, as The Wall Street Journal details:

Under the military agreement, Marines will rotate through existing Australian bases for joint training and exercises. The force will begin at 250 Marines, growing over several years into a full 2,500-person Marine Air Ground Task Force. Officials said the Marine deployment would come from existing forces and not require new spending.

The plan was also seen as a way to reassure allies and show that any domestic budget belt-tightening wasn't going to affect our national security priorities, according to Reuters:

[S]ome Asian nations are likely to welcome the U.S. move as a counterbalance to China's growing military power, especially its expanding maritime operations, and a reassurance that Washington will not scale back its engagement in the region due to a stretched U.S. military budget.