Falls under “freedom of overflight,” which necessitates that when entering another country’s EEZ you must
shall have due regard to the rights and duties of the coastal state
shall comply with the laws and regulations adopted by the coastal state
Per these stipulations, overflight is meant to be for peaceful purposes, not for e.g. spying, military coercion, etc. These are, more or less, the same conditions granted for innocent passage. Moreover, overflight often requires clearing with the government as per local laws and regulations.
There’s an open debate in international law as to whether surveillance is considered a peaceful purpose… But if someone dropped sonar buoys (“surveillance”) 12 miles out of Honolulu Harbor, you’d probably throw a fit.
Again, as I’ll repeat, flares are literally described by the FAA as a way to intercept aircraft. The US has used them against Russian aircraft, but in fact the US also does so to intercept civilian aircraft!
Current statements are that the helicopter was in “breach of Chinese EEZ” (Peter Cronau, ABC) and “within close range of Chinese airspace” (Chinese Foreign Ministry). Notably, the Australians have ignored calls by the British to release videos or GPS data on the incident.