'Emergency incident' unfolding at Sydney Airport

"An update will be provided at an appropriate time," the spokeswoman said.

There are reports a passenger is being unruly inside a plane stopped on the tarmac.

Malaysia Airlines flight MH122 left for Kuala Lumpur at 1.06pm but landed back on the Sydney runway at 3.47pm, flight tracking website FlightAware shows.

An airport spokesman said emergency agencies were being supported in their management of the incident.

"The airport is operational with flights arriving and departing," the statement said.

"Passengers are encouraged to check directly with their airline regarding the status of their flight."