Announcing the measure, Prime Minister Tony Abbott said he wanted “to stress that this does not mean that a terror attack is imminent” and that there were “no specific intelligence of particular plots”.

He said the decision was based on a number of factors including the number of Australians fighting in the Middle East and those who have returned home.

“What we do have is intelligence that there are people with the intent and capability to mount attacks here in Australia,” he said, adding that there were growing number of Australians “working with, connected to or inspired by” Islamist groups.

“We do know that people coming back from the Middle East militarised and brutalised … do pose a threat to our community,” he said.

“High” means a terrorist attack is likely and, as a result, Australians can now expect greater security at airports, large public events, government buildings and ports.

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