Thousands of passengers at Sydney Airport are still waiting to be processed through customs and security to board their flights after a power failure brought baggage and security processes to a standstill.

Power at the international terminal was lost for about an hour sometime after 9:30am AEST today.

Even though it has now been restored, international flights are expected to be delayed and it could be several hours before the backlog is cleared.

At the height of the disruption passengers were unable to check in or be processed through customs or security.

Sydney Airport corporate affairs manager Michael Samaris says the recovery is underway.

"Several dozen services will be impacted by this," he said.

"The recovery is underway. A number of flights have already taken off that were held up originally, so there has been delays, probably some more into this afternoon but the recovery is well and truly underway."

Passenger Dianne Rangi says people did not know what was going on.

Many other passengers criticised the airport's handling of the situation, and complained that they would miss connecting flights.

Others have complained about the chaos, saying no water was provided.

The Sydney Airport Corporation has apologised to the thousands of passengers whose flights have been disrupted.

Mr Samaris says an investigation is underway into the cause of the power failure, and into how well the episode was handled.

"These things are very difficult days. Sydney Airport always wants to provide a top level of passenger service and when something like this happens it's extremely regrettable that people are inconvenienced," he said.

"Sydney Airport apologises for the people who were inconvenienced in their travel plans today."