The Department of Defence said on May 26 an RAAF P-8 maritime surveillance aircraft was intercepted by a Chinese J-16 fighter during routine maritime surveillance activity in international airspace in the South China Sea region.

"The intercept resulted in a dangerous manoeuvre which posed a safety threat to the P-8 aircraft and its crew," it said.

The prime minister said the government has expressed its concerns with the Chinese government through appropriate channels.

"I won't be making further comment on it, other than to say that in the Australian government's view, in the Defence Department's view, this was not safe, what occurred," he told reporters in Perth on Sunday before embarking on an official trip to Indonesia.

Deputy Prime Minister and Defence Minister Richard Marles said the Chinese J-16 aircraft flew very close to the side of the Australian plane, where it released flares.

"The J-16 then accelerated and cut across the nose of the P-8, settling in front of the P-8 at very close distance," he told reporters in Melbourne.

"At that moment, it then released a bundle of chaff, which contains small pieces of aluminium, some of which were ingested into the engine of the P-8 aircraft. Quite obviously, this is very dangerous."

He said the crew of the P-8 responded professionally and returned the aircraft to its base.

The department said Defence has for decades undertaken maritime surveillance activities in the region and does so in accordance with international law, exercising the right to freedom of navigation and overflight in international waters and airspace.

"We're operating completely within our rights ...most of our trade traverses the South China Sea," Mr Marles said.

"This incident will not deter Australia from continuing to engage in these activities, which are within our rights and international law to assure that there is freedom of navigation in the South China Sea, because that is fundamentally in our nation's interest."

Opposition Leader and former defence minister Peter Dutton said the incident was concerning.

"This is a very serious time, and we will support the government in whatever actions they need to take to keep our country safe," he told reporters in Brisbane, where he was announcing his coalition frontbench.

"An act of aggression, as we have seen here, is not dissimilar to what we have sent on a regular basis in the East China Sea."

He has appointed Western Australian MP and former SAS member Andrew Hastie as opposition defence spokesman.

Australian Associated Press