Was it a confrontation on the high seas, or just a routine but unplanned interaction between warships sailing in international waters?

There are varying accounts within defence circles over just how stern a recent encounter in the South China Sea was between the Royal Australian Navy (RAN) and the People's Liberation Army-Navy (PLA-N).

On Thursday the ABC revealed an Australian Defence Force Joint Task Group had traversed the hotly contested waters last week, en route to the Philippine Sea for training exercises with the US and Japanese navies.

The Defence Department still won't even formally confirm that the five Australian warships interacted with the Chinese military but has insisted that "unplanned interactions with foreign warships throughout the deployment were conducted in a safe and professional manner".

According to one senior official the Chinese were "exceedingly polite" as they reminded the Australians they were coming close to the Spratly Islands which have been heavily fortified by China in recent times.

It's by no means the first time the ADF has been challenged by the Chinese in the area, but the encounter comes during a period of escalating security and diplomatic tensions between Australia and its largest economic partner.

Now Australia has dramatically raised the stakes in its already troubled relationship with China by backing the United States in formally declaring Beijing's territorial claims in the South China Sea to be illegal.

In a letter to the United Nations, Australia's permanent mission rejected the Chinese Communist Party's claims to disputed islands in the crucial trading waters, calling them "inconsistent" with international law.

Australian National University International law expert Professor Donald Rothwell believes the move is significant and will prompt a furious response from Beijing.

"I think what will be interesting to see is whether China will take a more assertive position in terms of physically challenging the rights of Australian warships in particular as they pass through the South China Sea," Professor Rothwell tells the ABC.

Richard McGregor, a senior fellow with The Lowy Institute, says the stakes are already high in the strategic and highly militarised corridor.

"You can absolutely be sure that any time Australian ships are in the South China Sea, they will be tracked by the Chinese," he told Radio National on Thursday.

"I don't think confrontation is the right word, but they will be hailed, they'll be asked what they are doing there and [asked] to explain themselves."

For the past few years defence officials have watched with increasing nervousness as Beijing has steadily built up weapons and runways on its disputed islands in the South China Sea, while also constructing artificial outposts for military purposes.

Unlike the United States, Australia does not conduct Freedom of Navigation Operations (FONOPs) to challenge territorial claims in the South China Sea, meaning the RAN takes care to stay outside the 12 nautical mile limits imposed around Chinese claimed territory.

Australia, however, continues to assert its right of freedom of navigation and overflight in the region by regularly flying and sailing military assets through the crucial trading zone.

On Tuesday evening as HMAS Canberra, and four other Australian warships were conducting military drills in the Philippine Sea, the US Defence Secretary Mark Esper was promising a record number of FONOPS against China would continue.

"We want to deter against coercive behaviour by the Chinese in the South China Sea," he told the International Institute for Strategic Studies.

"I am concerned that while the United States and our partners focus on supporting one another during these challenging times, the Chinese Communist Party continues to engage in systematic rule breaking, coercion and other malign activities."

Far from slowing China's activities, this year's coronavirus pandemic has had the effect of giving cover to the People's Liberation Army as it keeps up the rapid tempo of its territorial expansion in the region.

It's now widely accepted in Australia and across this region that Beijing has essentially established military control over the vast South China Sea, and efforts to challenge their authority may even work to enforce that view.

Secretary Esper has nevertheless warned that the United States will continue to conduct FONOPS in defiance of Beijing's territorial claims.

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"In 2019 we conducted the greatest number of Freedom of Navigation Operations in the South China Sea in the 40-year history of the FONOPS program, and we will keep up the pace this year," he said.

In Canberra it's no secret the Americans would dearly love their Australian allies to also conduct FONOPS, but there is no appetite within the Federal Government to do so.

Instead the ADF is committed to building up regional partnerships and remains hopeful that Australia will soon be invited to re-join the Malabar military exercises involving the US, India and Japan.

Top officials believe there are "very positive signs" an invitation will soon be formally issued, but in the meantime the so-called Quad looked to be on display last week as the United States conducted simultaneous exercises with Australia and Japan, and separately with India.

Now attention is turning to how soon Beijing could move to establish an Air Defence Identification Zone (ADIZ) over the disputed territory to monitor and control foreign aircraft.

Reports from China suggest planning for an ADIZ in the South China Sea are well advanced, similar to the one Beijing announced for the East China Sea in 2010, and then introduced in 2013.

In the meantime, there are growing concerns that the increasing presence of the US Navy in the region is heightening tensions that could spark actual conflict.

"Generally speaking the chances of some kind of conflict in the South China Sea are rising," Richard McGregor says.

And when the world's two largest superpowers are facing off in this region, the Australian Defence Force is acutely aware that a small miscalculation could have enormous consequences.

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China has won control of the South China Sea. Now we wait for Beijing's next move - ABC News