The heightened alert after years on “medium” officially means a “terrorist attack is likely” and comes after repeated government warnings that attacks could happen.
“There are people with the intent and the capability to mount attacks here in Australia,” Prime Minister Tony Abbott told a press conference.
The lifting of the threat level was “not based on knowledge of a specific attack plan but rather a body of evidence that points to the increased likelihood of a terrorist attack in Australia”, Abbott added in a joint statement with Attorney-General George Brandis.
“Security and intelligence agencies are concerned about the increasing number of Australians working with, connected to, or inspired by terrorist groups such as ISIL (Islamic State), Jabhat Al-Nusrah, and Al-Qaeda,” he said.
“The threat they pose has been increasing for more than a year.”
The “high” alert is just below “extreme,” the top level, which would indicate a “terrorist attack is imminent or has occurred”.
It followed a similar decision by Britain in August, which raised its terror threat risk level to “severe” meaning an attack is thought “highly likely”.
Abbott said his country’s alert was now “broadly comparable to the threat level in the United Kingdom”.
The government believes up to 60 Australians are fighting alongside jihadists for Islamic State, while another 100 were actively working to support the movement at home.
Abbott said based on previous experience with Australians who went to Afghanistan and Pakistan to join the Taliban, they had retained the “inclination to engage in terrorist activity” when they returned home.
“We do know that people coming back from the Middle East are militarised and brutalised, accustomed to kill without compunction, do pose a significant threat to our community should they not be under the closest possible supervision,” he said.
Abbott said the new threat level was not linked to his government’s decision to support US military action in Iraq and Syria, adding: “We have experienced significant levels of threats for a long time now.
“We would be targets regardless of anything that we did.”
Brandis stressed that the decision was not targeted at the country’s Muslims.
“This is not about any particular community, but it is a sad fact that the wicked people who recruit and exhort to violence Australian citizens are preying on the Muslim community so as to encourage their young men to fight… in the Middle East,” the Attorney General added.
“The steps that we have taken, the legislation we are preparing in consultation with that community in particular, is designed to protect them… and the interests of their communities.”
The decision came two days after police raided an Islamic centre in Brisbane and charged two men for alleged involvement in recruiting and sending jihadist fighters to Syria.
Authorities have alleged the men, Omar Succarieh, 31, and Agim Kruezi, 21, have links to Al-Nusrah in Syria.
Abbott said Australians “should continue to go about their lives” despite the increased alert, although security would be stepped up.
“What people will probably notice is more security at airports, more security at ports, more security at military bases, more security at government buildings and more security at public events,” he said.
Canberra recently announced a Aus$630 million counter-terrorism package that would boost support for security agencies such as the Australian Federal Police, spy agencies and customs.