More than 25 firefighters are responding after a white substance was found in the cargo hold of an aircraft at Sydney Airport on Friday morning.
The spill was located while crew were unloading luggage from an incoming American Airlines passenger aircraft.
Hazardous materials crews were immediately called.
Firefighters identified the substance as a "low irritant chemical additive" and have handed it over to forensic services for further testing.
A NSW Fire & Rescue spokesman confirmed seven specialised hazardous materials crews were deployed to the tarmac.
"We have HAZMAT crews on the scene with spillage gear and detectors," the spokesman told Fairfax Media.
"They're running some tests to identify what the substance might be."
The spokesman said the priority was to "make the area safe".
"Our crews arrived on the scene just after 8am.
"It is a significant response for a hazardous materials response at a site like an airport," he said.
Sydney Airport website said American Airlines flight 73 arrived from Los Angeles, landing at 7.05am and docking at gate 9.
After a three-hour turnaround, the aircraft is scheduled to return to Los Angeles at 10.15am.
Sydney Airport referred the matter to Air Services Australia, which has been contacted for comment.
More to come