Speaking ahead of the three-day Shangri-La Dialogue in Singapore, Mr Marles said the meeting was an "incredible opportunity" for defence ministers and policymakers from around the world to exchange ideas and develop relationships.
"We've seen China engage in the biggest conventional military build-up in the world since the end of the Second World War, and that has not happened with a strategic reassurance for other countries," Marles told Reuters in an interview.
Officials in Beijing did not immediately respond to a request for comment.
For the second year running, China's Defence Minister Dong Jun has skipped the meeting and Beijing has said it planned to send a delegation consisting mainly of experts and scholars from the People's Liberation Army.
Marles, US Defence Secretary Pete Hegseth, Japanese Defence Minister Shinjiro Koizumi as well as counterparts from France, Britain, Malaysia, the Philippines and other nations are attending.
"(China's) presence is something that we welcomed in the past, and having opportunities to engage with China are important," Mr Marles said.
Australia, he said, was expanding defence ties with nations across the Asia-Pacific but the alliance with the US remained the cornerstone of Canberra's security policy.
Despite worries the US was distracted from the Asia region because of the Iran war, Mr Marles said: "We see that America remains very committed to the Indo-Pacific, and from our point of view, our alliance with the United States is absolutely fundamental to our national security.
"The global rules-based order is under pressure in the Indo-Pacific," he added, referring to the international system of shared laws, agreements, and institutions established after World War II.
"This is a moment in time where we are looking to all the relationships that we have around the world, where we have common ground and where we can work together and where we can, we do."
Marles, Hegseth and British defence minister John Healey have scheduled an announcement on Saturday on their AUKUS project, under which Australia will acquire nuclear-powered submarines.
Marles declined to give details, but media reports say the three countries are preparing to announce a significant collaboration on uncrewed underwater vehicles.
