Three Australian warships were challenged by the Chinese military as they travelled through the disputed South China Sea earlier this month, the ABC can reveal.

The confrontations with the People's Liberation Army are believed to have occurred as China was conducting its largest ever naval exercises in the hotly contested waters.

Defence sources have confirmed HMAS Anzac, HMAS Toowoomba and HMAS Success were challenged by the PLA Navy as they were transiting towards Vietnam where they are now conducting a three-day goodwill visit in Ho Chi Minh City.

One official, speaking on the condition of anonymity, insisted the exchanges with the Chinese were polite, but "robust".

The Defence Department refused to answer questions or discuss details of the interactions between the Australian warships and the Chinese military.

In a statement, it confirmed HMAS Anzac and HMAS Success recently travelled through the South China Sea after leaving Subic Bay in the Philippines, while HMAS Toowoomba also went through the disputed waters after departing from Kota Kinabalu in Malaysia.

"The Australian Defence Force has maintained a robust program of international engagement with countries in and around the South China Sea for decades," the department told the ABC.

"This includes bilateral and multilateral military exercises, port visits, maritime surveillance operations and ship transits."

Prime Minister Malcolm Turnbull would not confirm the incident but reiterated Australia's right to conduct freedom of navigation exercises in the area.

"We maintain and practise the right of freedom of navigation and overflight throughout the world, and in this context, naval vessels on the world's oceans including the South China Sea, as is our perfect right in accordance with international law," he said.

Defence Industry Minister Christopher Pyne also reiterated Australia's rights within international law and downplayed the seriousness of the incident.

"I think 'confrontation' is somewhat of a tabloid-style description of what goes on in the South China Sea very regularly," he said.

In a statement, China's Defence Ministry said: "“The reports from Australia are different from the facts".

"On April 15 China's naval vessels encountered Australian naval ships in the South China Sea. China's ships used professional language to communicate with the Australian side. China's operation is lawful and conforms to conventions. It is professional and safe."

During their port visit to Ho Chi Minh City, HMAS Anzac, HMAS Toowoomba and HMAS Success will conduct a logistic resupply and local engagement with Vietnam People's Armed Forces and local government officials.

Earlier this month, Beijing boasted of the largest-ever military exercises by the Chinese navy in the contested South China Sea.

In the rare display, China brought much of its naval hardware for the world to see — 10,000 personnel, 76 fighter jets, 48 naval vessels, a nuclear powered submarine and China's first aircraft carrier.