The operators of Sydney Airport have blamed Airservices for a "system issue" that grounded planes to and from Sydney this morning.
Some flights out of Sydney's domestic terminal are being delayed reportedly due to a problem with air traffic control systems.
Some flights were able to land in Sydney, but several outgoing domestic and worldwide flights have been delayed or cancelled after systems went down at about 5.20am.
Sydney airport was thrown into chaos on the first day of the NSW school holidays after a technical problem grounded all departing flights. "We are also managing flights safely", Air Services Australia spokeswoman, Sarah Fulton said.
The airport's digital flight display includes a message reading "delayed to ATC radar failure", according to a Twitter image.
Travellers have been urged to check with their airlines before arriving at Sydney Airport on Monday morning.
Passengers have been told the system fault is affecting flights around Australia. Planes due to fly from Melbourne to Sydney remained on the tarmac and delays were expected throughout the day, a Melbourne airport spokesman said.