Perth's international airport terminal is in lock down this afternoon over the discovery of an unattended item, throwing passenger travel plans into chaos.

WA Police and Australian Federal Police officers locked down part of Terminal 1, the international wing of Perth Airport, and have established an exclusion zone while the item in the forecourt is investigated.

Passengers returning from overseas have faced long stagnant queues since the middle of the afternoon as they wait to clear customs.

One told The West Australian they had been waiting close to two hours, since their flight from Bali had landed.

"The line was long but was moving," they said.

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"At perhaps 4pm the line stopped moving - zero announcements on PA, zero communication from any staff until perhaps 4.50pm.

"Then about 5pm some staff member finally came and told us there was an AFP incident out front and they're not letting us out until its cleared."

A Perth Airport spokesperson confirmed the eastern end of the forecourt has been locked down, but said travellers can exit using the western end.

Traffic outside is being directed to use the Terminal 2 short term carpark, due to the exclusion zone making the regular area unusable.

"Check in areas at Terminal 1 International are currently closed to passengers and staff, and the Terminal 1 short term car park has also been closed, with traffic redirected to the Terminal 2 short term car park," the spokesperson said in a statement.

"Arriving international passengers will be escorted from the terminal via alternative doors.

"Perth Airport is working closely with the AFP and WAPOL to maintain the exclusion zone while the investigation continues."

More to come.

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Traveller chaos as police put Perth Airport into lockdown