Jetstar passengers are being warned to expect lengthy delays if they're heading to the airport, with an IT outage disrupting check-in counters across the country.

Passengers are facing delays at multiple airports, including Melbourne and Brisbane, after a reported mass outage.

"An IT issue is affecting some of our airport systems today, resulting in a number of delays and some cancellations to our domestic and international flights," a spokesperson for Jetstar told news.com.au.

"Our technicians are working to resolve the issue as soon as possible. We appreciate delays are frustrating and apologise to customers for the impact to their journey."

One passenger claimed they had been waiting for three hours for their flight, while others said they had been waiting to board since 6am.

"Short delays bulls**t. My flight was supposed to take off at 7am from Brisbane but we can't even check-in yet," another added.

"9.15 JQ457 Ballina to Sydney estimated departure 13.15. Four hours is not short," another added.

It is understood groundstaff are unable to access IT systems, meaning logins are unavailable with passengers being urged to log on themselves and change their flights.

The lengthy delays come during a testing time for Jetstar, which later this week will have staff - including baggage handlers and ground workers - walk off the job.

On Friday, more than 250 Jetstar staff will walk off the job during busy periods as the first wave of strike action hits the airline.

Transport Workers Union leaders have told the company about the planned stoppages after a majority of its members voted to launch protected industrial action.

Crews at Melbourne Airport will strike from 9.30am to 11.30am and again from 4pm to 6pm. In Sydney, staff will walk off the job between 5.30am to 7.30am and 5.30pm to 7.30pm, while in Brisbane and Adelaide there will be a strike from 6am to 8am and 6pm to 8pm.

More to come