Amid heightened concern about Beijing's growing clout and military muscle flexing, the appearance of a Chinese flagged task group and around 700 sailors came as a surprise.

"It may have been a surprise to others, but it certainly wasn't a surprise to the government," said Prime Minister Scott Morrison, trying to dampen concerns.

"We have known about that for some time," he said of the visit during a trip to the Solomon Islands.

Morrison described it as a "reciprocal visit because Australian naval vessels have visited China."

"They were returning after a counter drug trafficking operation in the Middle East."

The vessels appeared to be the Kunlun Shan, a Yuzhao class landing ship; the Luoma Lake replenishment ship and Xuchang, a modern frigate that is believed to be fitted with surface-to-air and anti-submarine missile systems.

The timing of their visit has also been questioned.

It comes on the eve of the 30th anniversary of the violent suppression of Tiananmen protests.

Then, the regime gunned down hundreds of its own citizens and jailed thousands for demanding political change and an end to state corruption.

The sailing also comes just days after it was revealed that a Chinese warship had recently confronted an Australian vessel in the South China Sea and Aussie helicopter pilots had been targeted with lasers.

"I think any reading into timing could be subject to a bit of overanalysis," said Morrison.

Since coming to power, President Xi Jinping has invested heavily in the People's Liberation Army Navy -- in a bid to project Chinese influence across the Pacific and beyond.

"Chinese naval visits to Australia have more typically been a lone frigate, not a task group with an amphibious assault ship and 700 personnel," Rory Medcalf, head of the National Security College at Australian National University, tweeted.

"Sydney is hardly a convenient stopover on their way home from the Gulf of Aden. What's the story here?"