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A group of Chinese navy vessels, including a formidable warship, sailing legally in the Tasman Sea have raised alarm in Australia and New Zealand because they were in unusually southern waters and on an undeclared mission.

Australian officials said Thursday that they were closely monitoring the ships — a cruiser, a frigate and a supply vessel. They have been tracking them since last week, when they were detected off Australia’s northeast coast.

This week, the three ships were traveling within about 150 nautical miles of Sydney, outside Australia’s territorial waters but within its exclusive economic zone, according to an Australian government official speaking on condition of anonymity. Their presence near Sydney was first reported by The Financial Times.

China’s military has not publicly commented on the naval vessels, and the Chinese Embassy in Australia did not respond to a request for comment.

Richard Marles, Australia’s defense minister and deputy prime minister, said that the ships were operating in accordance with international law, and that the Australian military was closely observing their activities from the sea and from the air. He said in a television interview that the presence of the Chinese navy in the area was “not unprecedented,” but “unusual.”

“We are keeping close watch on them, and we will be making sure that we watch every move,” he said.

Chinese warships docked at Sydney Harbor in 2019, in what the prime minister at the time called a “reciprocal visit” after Australian navy ships had visited Chinese ports. This time, Australian officials said they were in the dark about where the ships were headed and what their objective was.