"While ATC operations are back to normal, impact will continue throughout the afternoon," the alert said.

"Please check with your airlines for flight status."

Airservices Australia's latest alert posted to Twitter said all restrictions have now been lifted at Sydney Airport following this morning's incident.

"We are fully back to operations at Sydney Tower. Departures are running smoothly," the alert stated.

The smoke alarms in the ground level of the Sydney Airport control tower went off at about 11.40am on Friday after smoke was detected, Fire and Rescue NSW told The Sydney Morning Herald.

ALL flights out of Sydney are being delayed due to an emergency situation in the airport's control tower. 

Airservices Australia Tweeted issued the alert at 12.03pm on Friday.

"The Sydney Air Traffic Control Tower has been evacuated after smoke was detected," the alert said.

"There is a full ground stop in place with no aircraft departing.

"We will update you as soon as we know more."

Initially, all arrivals and departures were delayed, however a recent update from Airservices Australia said arrivals are being processed, but aircraft were landing later than their expected arrival time.

Virgin Australia have also alerted their customers via social media.

"There are currently no aircraft flying in and out of Sydney Airport," they posed to Twitter.

"We will continue to provide updates as more information becomes available.

"To check your flight status, click here."

More to come.