Sydney, Mar 29 (IANS): The Sydney Airport's control tower was evacuated on Friday after a computer battery back-up system overheated, initially preventing all planes at the airport from departing and significantly delaying inbound flights from landing.

Flights were delayed by up to two hours while some inbound planes were diverted to other Australian cities of Canberra and Melbourne, The Sydney Morning Herald reported.

At about 11.40 a.m., fire alarms went off and smoke was detected on the ground level of the control tower, Fire and Rescue New South Wales Inspector Bryce Jonas told The Herald.

"Occupants could smell smoke, they did the right thing and started to evacuate after diverting air traffic control to Melbourne," Jonas said.

He said thermal energy detectors were used to identify the source of the smoke and located a battery back-up system for the computers which malfunctioned and overheated.

"Firefighters used carbon dioxide extinguishers on the battery system to cool it down," Jonas added.

About 20 people were evacuated from the control tower. They returned within an hour of the evacuation.

Airservices Australia said at 1.19 p.m., the tower was "fully back to operations".

The Sydney Airport, which is a hub for airlines including Qantas Airways and Virgin Australia, is the nation's busiest airport.