Sydney airport customs building evacuated

A customs office at Sydney airport has been evacuated with about 40 people forced from a building where smoke is coming from an unknown source.

Authorities were called about 8.30pm on Sunday after an automatic alarm went off and three NSWFRS engines and an airport vehicle are at the scene.

"On arrival they noticed smoke coming from the building," a NSWFRS spokesman said.

"Firefighters are still checking the source of the smoke. They have not located that."