Passengers have been left stranded and shops unable to sell after a power outage caused "a big mess" at Sydney airport.

Duty free shops were forced to stop trading for most of the day, after power was lost at the international terminal from about 9:40 a.m. to 11:00 a.m. (AEST).

"It was a big mess," a sales assistant at an airport duty free store said.

"Even after the lights came back on, we had to stay closed because the computers weren't working," she said.

Hours later, at 4:30 p.m., hundreds of suitcases belonging to passengers had been cordoned off at the Qantas check-in area and were being guarded by security staff because carousels weren't working.

Jetstar also had luggage cordoned off.

Throughout the day, security screening, check-in and baggage-handling procedures were affected, resulting in dozens of flights being delayed, Michael Samaras, the spokesman for Sydney Airport Corporation, said.

Back-up generators also took some time to come on, compounding the problem.

Stranded passengers voiced their frustration on Twitter.

"Absolute chaos at Sydney airport. Am definitely going to need that massage when I get to Bali," one passenger wrote on the microblogging site.

"Check in isn't working, power outage, computers down. Line hasn't moved for 2 hours," wrote another, while another said: "Someone obviously forgot to pay the electricity bill."

Airport sources said the blackout was caused by a fire at the power station. However, Mr Samaras said this could not be confirmed.

"An investigation into the outages is under way. But right now, the focus is still on recovery," he said.

"Sydney Airport is now doing its utmost to return the terminal to 100 percent operation and is working closely with airlines, border agencies and government authorities to restore normal operations."