Defence has cited "operational security reasons" for not discussing an Australian warship's recent encounters with the Chinese military while sailing through international waters claimed by Beijing.
Military sources claim HMAS Parramatta has been closely tracked by the People's Liberation Army over recent weeks, including being followed by a Chinese nuclear-powered submarine, a warship and multiple aircraft.
"Formal challenges have occurred, such as telling us that we're entering 'China's territorial waters'," a Defence figure familiar with the interactions said.
"The most intense activity occurred as HMAS Parramatta was in the East China Sea," the official told the ABC, speaking on the condition of anonymity because they were not authorised to publicly discuss operations.
Since departing Australia in late May, the ANZAC-class frigate has travelled to Vietnam and South Korea and then on to Japan via the South China and East China Seas as part of a "regional presence deployment" which has included several joint military exercises with other navies.
According to the Australian embassy in Vietnam, HMAS Parramatta arrived in the port city of Da Nang on June 5, before departing the country over a week later bound for the Busan Naval Base in South Korea.
That trip took the ship through the South China Sea and then the East China Sea past Taiwan, which include heavily militarised territory claimed by the Chinese.
During its passage, the ABC was told, HMAS Parramatta was closely tracked by Chinese military assets including a Type 052C "Luyang II" guided-missile destroyer and a Type 093-A "Shang II" nuclear-powered attack submarine.
On June 28, the Australian warship then arrived in the Port of Sasebo, in the Nagasaki prefecture, after completing naval exercises with Japan's Self-Defence Force.
Last week the ABC approached the Defence Department with a series of detailed questions about the Chinese military's interactions with HMAS Parramatta, but it declined to answer them.
"HMAS Parramatta is 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 departmental spokesperson said.
"Regional deployments form part of Australia's longstanding contribution to an open, inclusive and resilient Indo-Pacific.
"For operational security reasons, Defence does not publicly disclose specific details of operations," the spokesperson added.
Defence Minister Richard Marles, who is visiting the United States, also declined to comment when asked by the ABC about the recent encounters.
"I won't go into details around that, other than to say that what our military does, what our navy, does in the South China Sea is routine," Mr Marles told the ABC from Washington DC.
"It's been doing it for decades, and it is focused on asserting the UN Convention on the Law of the Sea: freedom of navigation, freedom of overflight, the global rules-based order, which I've been describing as being so important for our national interest.
Asked whether the government may choose to later publicly disclose the incident as it has with previous encounters with the Chinese military, Mr Marles left open the possibility.
"That may happen in the future again, but what we are focused on first and foremost is doing the activity because that's in our national interest."
"And then obviously, in terms of the management of the information around that activity, we're focused on the safety of our servicemen and women."
Last week Japan's Maritime Self-Defence Force (JMSDF) announced it had "conducted a trilateral exercise with United States Ship Dewy and HMAS Parramatta in the East China Sea to East of Okinawa" between July 4 and 6.
"The Japan Maritime Self-Defence Force strengthens cooperation among JMSDF, US navy and Australian navy in order to realise a free and open Indo-Pacific," it said.
Earlier this year the Defence Department revealed a Chinese J-16 jet fighter had flown close to an RAAF P-8 maritime surveillance plane during a routine patrol in the South China Sea.
On Friday Foreign Minister Penny Wong met her Chinese counterpart, Wang Yi, in Bali, the first such face-to-face meeting at such a high level in almost three years.
