An airport passport control system has been restored after an earlier outage caused delays at international airports around Australia and New Zealand on Monday.

The outage hit the Advanced Passenger Processing system, which is used during check-in to confirm whether a passenger is authorised to travel to their destination, Melbourne Airport's head of communications Grant Smith said.

The system crashed about 8am, forcing airlines to check passengers in manually and causing departure delays on international flights of up to an hour.

The passport system was restored at Sydney and Melbourne airports just before 11am. By midday, it appeared most flights were departing on time.