China’s military issued “robust” challenges to three Australian warships as they travelled through the South China Sea to Vietnam earlier this month, the ABC reports.

Defence sources said the confrontations between HMAS Anzac, HMAS Toowoomba and HMAS Success and the People’s Liberation Army occurred ahead of the Australian vessels’ arrival for a three-day goodwill visit in Ho Chi Minh City.

One official, speaking on the condition of anonymity, told the ABC exchanges with the PLA navy were polite but “robust”.

The Australian prime minister, Malcolm Turnbull, would not reveal what happened in the exchange with the Chinese military when questioned about it on the sidelines of the Chogm meeting in London on Thursday, local time.

“All I can say to you is Australia asserts and practises its right of freedom of navigation throughout the world’s oceans, including of course the South China Sea,” Turnbull said. “As is our perfect right in accordance with international law.”

The defence department confirmed the three vessels recently travelled through the South China Sea but refused to provide details of the interactions between Australia’s warships and Chinese military, the ABC reported on Friday.

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

“As they have done for many decades, Australian vessels and aircraft will continue to exercise rights under international law to freedom of navigation and overflight, including in the South China Sea.”

China recently completed a massive military drill in the South China Sea, where its claims are hotly disputed by Vietnam as well as the Philippines, Malaysia, Brunei and Taiwan.

China’s construction of islands and military facilities in the South China Sea, through which some $3tr in trade passes annually, has sparked concerns Beijing is seeking to restrict free movement and extend its strategic reach.

The United States has conducted “freedom of navigation patrols” through the South China Sea, stoking tensions with China which says it will protect its sovereignty.

The Australian navy ships Anzac, Toowoomba and Success are on a three-month deployment in south-east Asia, which will involve exercises with a number of countries in the region, the defence department said in a separate statement on 17 April.

The Toowoomba sailed to Vietnam from Malaysia, while the other two Australian warships went through the South China Sea from Subic Bay in the Philippines.