The travel plans of thousands of Australians were disrupted today by a power outage at Sydney's international airport.

Australia's busiest air terminal was blacked out for an hour and a half, crippling security screening and check-in and delaying "thousands" of passengers, a Sydney Airport spokesman said.

"The cause of the failure is under investigation," he told AFP, estimating that it would take "several hours" to get back on schedule.

Back-up generators also took "some time" to come on, he added, compounding the problem. The reason for the generator problems was also being investigated, he said.

Passengers said the outage brought customs and security to "a standstill" and there was "chaos" in the terminal, with huge lines as people were manually processed.

"Madness at Sydney Airport! Nobody going anywhere!" one passenger wrote on Twitter.

"Absolute chaos at Sydney Airport. Am definitely going to need that massage when I get to Bali ... if I get there, that is," another said.

Hundreds of people were left stranded on the tarmac, tantalisingly close to gangways left frozen by the blackout.

"Sitting about 20 feet from Sydney gate waiting because of power outage at airport -- 2.5 hours late and counting!" one man tweeted from his jet seat.