Would-be commuters were left stuck at check-in early on Friday morning after a technical issue resulted in the security screening being unable to open.
The airport is yet to estimate when the issue will be completely resolved.
A security incident is causing significant delays at Brisbane Airport's global terminal.
Qantas' domestic Terminal 3 was not affected.
Jetstar expected the issue to create a backlog of up to two hours throughout the day.
Virgin Australia said that "due to a technical issue" passengers were being processed at reduced capacity.
"We're working to resolve the issue as quickly as possible and thank you for your patience".
"We're advising passengers and visitors to avoid T1 and T2 until further notice", a Tweet from Sydney Airport said at 6:08am.
"The AFP is assisting Brisbane Airport security staff following a routine passenger screening matter", a statement from the Australian Federal Police said.
Sydney Airport said the technical issue was affecting passenger processing at the T1 worldwide and T2 Domestic terminals up until about 8:30am.
Officials are urging any passengers who planned to fly from Sydney airport today to call their airline and/or check their flight time and status online.
Many passengers have taken to social media to share their frustration.