Perth, Aug 3 (IANS): Australian Prime Minister Malcolm Turnbull announced on Thursday his decision to lower a terror alert at airports following a raid that led to the arrest of four persons who were allegedly plotting an attack on an airplane.

The threat level "was raised last week to 'probable' and has now been restored to its previous level", Turnbull said at a joint press conference with Justice Minister Michael Keenan here.

The threat level at airports was raised on advice from the country's security and intelligence agencies after the alleged attack was thwarted, reports Efe news.

Australian authorities are, however, planning to expand security measures to prevent any possible attacks on civil aviation.

"Some of them will be obvious to travellers," Turnbull said.

Last week, the Australian police arrested four persons, one of whom was released without charges, from the suburbs of Sydney.

The three persons, who are still in custody -- Khaled Khayat, Mahmoud Khayat and Khaled Merhi -- are believed to have links with the Islamic State terror group.

The Australian police suspect the detainees planned to hide a bomb in a kitchen meat grinder and smuggle it onto an Etihad Airways flight.

Australia raised its terror alert in September 2014 and has passed a series of anti-terrorist laws to prevent attacks on its territory.

The country has witnessed four violent incidents since then and security forces have disrupted 13 planned attacks and arrested 70 people in 31 operations.