Australia said Thursday it was "optimistic" about the emergence of China as a world power but said strong and peaceful ties between Beijing and the United States would be key to regional stability.

Defence Minister Stephen Smith said the US, China and India would be the world's three "super powers" by the second half of this century and Australia wanted Beijing to play a "full and constructive role" in the region.

"I am, and the government is, optimistic about the emergence of China," Smith said following a major defence policy speech in Sydney.

"We're positive and optimistic about that, but we do think it is crucial that China and the United States have a deep level of engagement, and the challenge now (is) to avoid strategic competition between (them)."

Australia has important ties with both countries -- China, a major market for its natural resources, is its top trading partner, while it has a long-standing military alliance with the US that has just been expanded.

Some 2,500 US Marines will be stationed in northern Australia in years to come and deeper aerial and naval co-operation is being planned as Washington makes a major shift towards the Asia-Pacific region.

Smith said there was "nothing inconsistent with a comprehensive and growing relationship with China and our military alliance with the United States", playing down Australia's importance in the historic pivot of power eastwards.

"Two great powers do not need a country with a population of less than 25 million people to be a bridge between them," he said in his speech to the Lowy Institute, a foreign policy thinktank.

"That is a matter for them -- and a positive constructive relationship between two such powers is essential for stability and security during this century."

Fears about the US presence in Australia were really just fears about US-China ties, he added, "and that's why we say to China, and... the United States the most important thing is to get your bilateral relationship right".

General James Amos, Commandant of the US Marines and one of the Joint Chiefs of Staff, visited the first deployment of troops to Australia's Northern Territory Thursday and said the move was not designed to intimidate China or others.

"I don't look at it as sabre-rattling at all," Amos told reporters in Darwin.

"There is a lot of opportunity to work together: humanitarian assistance, disaster relief, training together and to actually have an influence on... responsible behaviour in the Asia-Pacific area."

Smith said the Marines' presence had already seen talk of trilateral humanitarian and disaster drills with Indonesia, and Jakarta had signalled that "such exercises could be observed by or involve China in (the) future".

Though China and India would rise, Smith said Australia believed the US would "continue to be the single most important strategic actor in our region for the foreseeable future", both independently and through its alliances.