Three Chinese warships conducted a series of naval drills last Friday and Saturday, stationed underneath a busy flight path linking Australia and New Zealand.

Australia's air safety agency said it first learned of the drills when a commercial flight picked up a broadcast from the Chinese boats on Friday morning.

"At that stage we didn't know if it was a potential hoax or real," Air Services Australia deputy chief executive Peter Curran told a government hearing on Monday evening.

The warning was broadcast on a frequency monitored by commercial pilots -- but not by Australia's air traffic controllers, Curran said.

"It's an international guard frequency. Air traffic control does not monitor that frequency but pilots do. So we can't hear what was said."

Curran said 49 commercial flights were forced to divert around the live firing zone once it became apparent the warning was legitimate.

"Some of those were aircraft that were in the air at the time we first became aware of it."

Australia said the drills took place in international waters, and has conceded China's conduct abided by international law.

But it has criticised Beijing for running the exercises without appropriate warning.

China has defended its conduct as "safe, standard and professional".

Australia and close ally New Zealand have been monitoring the vessels -- a frigate, a cruiser and a supply tanker -- since they were spotted off Australia's shores last week.

The warships were 218 nautical miles east of Australia's island state of Tasmania on Tuesday morning, New Zealand's defence force said.

Canberra has irritated Beijing with its own navigation exercises in flashpoint regions such as the Taiwan Strait.

A Chinese fighter jet earlier this month dropped flares in the path of an Australian air force surveillance plane patrolling the contested South China Sea.

China's foreign ministry said the Australian plane was an intruder flying through the region "without Chinese permission".