Two Australian warships were closely tracked by the Chinese military last month as they passed some of Beijing's most heavily fortified islands in the South China Sea, as part of a joint show of force with the US and Japanese navies.
Officials with knowledge of the operation say destroyer HMAS Hobart and auxiliary tanker HMAS Stalwart sailed close to the Spratly Islands and were challenged by the People's Liberation Army (PLA) in international waters claimed by China.
The interaction with the PLA occurred during a "Regional Presence Deployment", where the Australian Navy was accompanied by the Japanese destroyer JS Kirisame and US Navy destroyer USS Milius for a trilateral transit of the South China Sea.
One defence figure, speaking to the ABC on the condition of anonymity, said the Chinese military closely tracked the vessels and showed a "significant increase in tactical acumen and use of EM (electronic warfare) capabilities".
"They (the Chinese) are being less overt, apathetic vis-a-vis their usual actions, behaviours and methodologies," the ADF figure claimed.
Foreign interactions 'safe and professional'
The defence department has declined to answer questions about the events citing "operational security reasons", but insists any "interactions with foreign militaries were conducted in a safe and professional manner".
"HMA ships Hobart and Stalwart are currently undertaking a Regional Presence Deployment, conducting a number of navy-to-navy activities with Australia's regional partners and participating in various maritime exercises," a Defence spokesperson told the ABC.
"Regional deployments form part of Australia's longstanding contribution to an open, inclusive, and resilient Indo-Pacific.
"During the deployment HMA Ships Hobart and Stalwart conducted various exercises through and within international waters in the South China Sea.
"All were in accordance with international law, including the United Nations Convention on the Law of the Sea (UNCLOS)."
Speaking in Perth, Assistant Defence Minister Pat Conroy wouldn't confirm the report, but reminded all countries of their obligations under International Maritime Laws.
"We call upon all nations to respect the laws of the sea and … we need to make sure that is followed, and that everyone is conducting themselves in a way that is safe for all involved," he said.
HMAS Hobart and HMAS Stalwart remain on deployment, and last week took part in the Japanese Maritime Self-Defense Force's International Fleet Review off the coast of Yokosuka.
Analysis of satellite imagery from the past year reveals hundreds of Chinese military vessels continue to operate in the Spratly Islands daily, while recent high-definition photographs have detailed radar installations, airfields and naval gun emplacements among other structures built there.
Earlier this year the ABC revealed frigate HMAS Parramatta was closely tracked by the PLA, including being followed by a Chinese nuclear-powered submarine while travelling through contested areas of the East China Sea and South China Sea.
At the time, the Defence Department and Defence Minister Richard Marles also declined to answer questions or provide any details of the interactions between the Chinese military and HMAS Parramatta.
