SYDNEY - Sydney Airport resumed operations on Friday (March 29) after air traffic control staff who were evacuated due to smoke were cleared to return to work.

Air Services Australia, the national air navigation service provider, said departures at the nation's busiest airport were running smoothly and all restrictions had been lifted.

"Air traffic control operations have resumed in Sydney, however delays are expected for the remainder of the day. Please check with your airline for flight status," Air Services Australia said in a tweet.

Sydney Airport is a hub for airlines including Qantas Airways and Virgin Australia, both of which said they were working to get flights back on schedule.

Departures were grounded and arrivals were delayed after as many as 20 control tower staff were evacuated earlier on Friday due to smoke from an overheating computer battery system, which then spread through the air-conditioning, reported the Australian Broadcasting Corporation (ABC).

New South Wales Fire and Rescue Superintendent Bryce Jonas told news.com.au firefighters were called to reports of a "nasty, electrical burning smell" in the air traffic control tower just before 12pm.

"The internal components of the battery pack system had started to smoke which created that nasty, electrical burning smell," Mr Jonas was quoted as saying.

Numerous passengers were held up in their planes during the incident.

"So I'm sitting on the plane on the runway at Sydney Airport... apparently there's smoke coming out of the Control Tower, Airport is in lockdown... nothing coming in or going out!" said Ms Suzanne Paul on Twitter.

Another affected passenger, Mr Tim Napier, tweeted: "Sitting on plane, captain says smoke in @SydneyAirport control tower, fire trucks on scene. Hope everyone is ok but may mean a slow trip home."