I’m actually glad you asked. The area where the Australian helicopter was intercepted was in China’s EEZ, near China’s territorial sea.
Normally, there are no restrictions for aircraft’s innocent passage through another country’s EEZ. This statement holds for the Yellow Sea, the Taiwan Strait, the South China Sea, etc.
However, this is true specifically for innocent passage and overflight. Australia claims they were in China’s EEZ to enforce UN sanctions on North Korea. The UN panel for monitoring sanctions on North Korea was disbanded last month, so Australia lacks the UN mandate to justify their presence. Moreover, spying monitoring is very distinctly not innocent passage, and sailing an armed vehicle through is also distinctly not innocent passage. Moreover, the Yellow Sea has no real candidate places to pass to, so Australia is hardly just passing through.
Of course, Australia could have also completed the entire mission within South Korea’s EEZ (a country that’s allied with Australia),
Or, put another way: if China parked their aircraft carriers 12 nautical miles from Taipei and started flying jets and helicopters within the EEZ but outside of the territorial sea, what do you think the response would be? If China launched a balloon that skirted around Taiwanese sovereign airspace but within Taiwan’s EEZ, what do you think the response would be? “Just let it slide”?