People's Liberation Army-Navy Renhai-class cruiser Zunyi is seen here in the Solomon Sea. (Supplied: ADF)
In short:
Defence officials believe a Chinese nuclear-powered submarine likely accompanied the People's Liberation Army-Navy flotilla during its deployment off Australia's coast.
It comes as commercial airline pilots who flew over the Tasman Sea last week told the ABC they monitored radio communications from the Chinese warships to military surveillance aircraft as far back as a week ago.
What's next?
Defence said it continues to "closely" track the Chinese flotilla, which had re-entered Australia's exclusive economic zone on Tuesday.Â
One of Australia's most senior intelligence figures says aspects of China's deployment of three warships to the Tasman Sea appear "designed to be provocative", as the naval task group continues to be closely tracked heading back towards Tasmania.
Director-general of the Office of National Intelligence (ONI) Andrew Shearer this week delivered a blunt assessment of Beijing's strategic aims for sending the heavily armed flotilla to the region, saying it was to "shape" the behaviour of states like Australia.
He also described its recent live-fire exercises as unprecedented and said they demonstrated "China's growing capability to project military power into our immediate region" was "now matched by an increasing intent to do so".
Appearing before a parliamentary committee on Monday night, the ONI boss observed this was "the furthest south a People's Liberation Army-Navy (PLA-N) task group has operated".
"We judge Beijing intends to normalise this sort of presence, shape the responses of those in the region, and observe and learn from our reactions," he told the Senate estimates hearing.
"The largest and least transparent military build-up since the Second World War will mean the PLA will be able to operate at greater distances from mainland China, in greater numbers, including into Australia's immediate seas and skies," he said.
On Tuesday, Defence confirmed the PLA-N flotilla had re-entered Australia's exclusive economic zone, where it was being closely tracked as it operated around 160 nautical miles east of Hobart.
One military figure, not authorised to speak publicly, told the ABC there was a "working assumption" that a taskforce comprising three warships so far from the Chinese mainland for weeks could have submarine support for at least part of the deployment.
People's Liberation Army-Navy Jiangkai-class frigate Hengyang in the Tasman Sea. (Supplied: ADF)
Other navies that boast nuclear-powered submarines, such as the United States and the United Kingdom, are known to regularly use them during lengthy and complex warship missions to gather valuable intelligence on potential adversaries.
Meanwhile, commercial airline pilots who flew over the Tasman Sea last week have told the ABC they monitored radio communications from the Chinese warships to military surveillance aircraft as far back as a week ago, warning about possible live-firing activity.
Chinese 'live fire' military exercise forced 49 flight diversions
Photo shows A Virgin plane flying in the sky in Brisbane.
"The Chinese vessel made a broadcast on 121.5 [VHF frequency] about a live firing drill, telling the NZ aircraft to remain at a safe distance," one international airline pilot told the ABC when describing a flight on Tuesday, February 18.
"The NZ aircraft replied that under international law they can be there, it occurred a couple of times. Many civilian aircraft heard it and told the Chinese to 'go away.'"
This week aviation officials revealed they were first informed that a Chinese live-firing exercise had begun when a Virgin Australia pilot relayed warnings they had picked up mid-flight via an emergency radio frequency.