UPDATE: NSW Firefighters have confirmed that the fire alarm in the Control Tower was triggered by a short circuit in the battery back-up of a computer system.
They told the Triple M Newsroom that the issue resulted in a small amount of smoke, which resulted in the evacuation of about 20 people.
The Tower is now being checked over with staff set to return shortly, and therefore flights as well.
Nine News are reporting that the control tower at Sydney Airport has been evacuated and is on lockdown.
As a results, all flights are grounded.
It has since been confirmed that smoke was detected in the Control Tower, and that "no flights are arriving or departing at this time".
More to come.