Thousands of passengers and staff had to be evacuated from Perth International Airport after several suspicious cardboard boxes were found on a plane from Malaysia.

The international terminal was shut down for almost 12 hours from 6pm on May 3 after baggage handlers noticed the packages on the incoming Malaysia Airlines flight from Kuala Lumpur.

The packages were eventually x-rayed by the bomb squad and found to contain nothing dangerous, but not before eight international flights were either cancelled, diverted or delayed.

Perth Airport chief executive Brad Geatches said it took about 20 minutes to evacuate some 3,000 passengers and staff to an exclusion zone 2km from the airport buildings.

Police also had to hold back thousands more people arriving at the airport to board flights or collect friends and loved ones.

Assistant commissioner of counter terrorism and state protection John McRoberts said on May 4 that it was one of the biggest security operations at Perth airport, involving about 100 officers.

"It's certainly the most significant event I know at the Perth airport," Mr McRoberts said.

"It was a major operation, it was an operation that caused ... considerable inconvenience to the travelling public, to the staff at the airport, to government agencies that responded, and that's regrettable."

"But I think people ought to take some comfort in the fact that the safety of everybody was of paramount concern to us."

Mr McRoberts would not explain what alerted police to the unaccompanied freight, but said they were part of a consignment destined for someone in Perth.

That person was spoken to by police last night and their story checked out with what police found when the boxes were x-rayed at a safe site on the airport grounds.

Police refused to say what was in the packages, fearing copycat events.

The airport was evacuated one hour after the packages were identified, but due to unexplained delays in the operation it took another nine hours before they were x-rayed.

Some passengers complained the evacuation had been poorly organised, but Mr Geatches described it as "successful from every measure".

He said these sorts of events unfold very rapidly and are extremely complex.

"You are dealing with a large number of people in the terminals, a large number of people arriving, you have a large number of aircraft at various stages of approach and departure from the airport," he said.

"These are very complex issues. I can assure you they were managed very professionally."

Mr Geatches also defended Kuala Lumpur airport, where the packages were loaded, saying he had every confidence in its security.

Perth International Airport was re-opened at 5.30am on May 4, with possible delays expected over the next two days.