"On the basis of recent events both here in Australia and overseas, and provided the information that the Prime Minister spoke about was sound and accurate there is a strong case to raise the terror threat to extreme as quickly as possible," he said. Prime Minister Tony Abbott says the National Security Committee met on Tuesday. Credit:Andrew Meares The alert from the Prime Minister comes as the NSW Opposition Leader John Robertson resigned hours after moves began to dump him over revelations he signed a letter of request for gunman Monis In recent days Mr Byrne has also called for a judicial inquiry into events surrounding the Martin Place siege and in particular whether or not the intelligence or security agencies could have done anything more to prevent the siege from taking place. Mr Abbott emerged from the security briefings in Sydney to front a snap media conference, in which he urged Australians to celebrate Christmas as normal but be on guard for any suspicious activity.

"The briefing from the security agencies today indicated that there has been a heightened level of terrorist chatter in the aftermath of the Martin Place siege," Mr Abbott said. "That's why it's important that people remain alert and aware." ASIO and the Australian Federal Police recommended the national terrorism threat level be raised from "medium" to "high" in September, meaning a terrorism attack was regarded as "likely". Cafe manager Tori Johnson and barrister Katrina Dawson were killed when Man Haron Monis took 18 people hostage in the Lindt Cafe on the morning of December 15 and held them for 17 hours. Mr Abbott confirmed on Sunday that an anonymous caller phoned the National Security hotline the weekend before the siege, to warn them of Man Haron Monis' extreme views.

Mr Abbott said the call was followed up but "didn't reveal any imminent attack by this individual." He stressed the Commonwealth-State investigation, due to report by the end of January next year, would look into the matter, as part of its examination into whether the policing and security agencies dealt with the matter to the best of their ability. Mr Abbott said he had attended a memorial service for Ms Dawson "a brilliant and beautiful woman from a wonderful and accomplished family who has been cruelly snatched away from us". "As we prepare to join family and friends to celebrate Christmas we do need to be conscious of the fact that the terror threat remains high. There are people who would do us harm and who can do us harm," he said. He urged members of the public to report any suspicious behaviour to authorities.

"I do want to reassure you though that our law enforcement agencies and our police forces and our security agencies will be working around the clock to keep you as safe as possible over the holiday season, as they do at all times." Mr Abbott said the national security committee of the cabinet met for the first time on Tuesday afternoon following the swearing in of his new ministers. "It was important to me that they were fully briefed on the domestic and international security situation as quickly as possible," he said. The national security committee was briefed by the head of the Australian Security and Intelligence Organisation and the commissioner of the Australian Federal Police. Asked if the Martin Place siege had provoked further security threats, Mr Abbott said he would not speculate.

"In the wake of the Martin Place brush with terrorism there has been a heightened level of chatter amongst people who we would normally think of as terrorist sympathisers," he said. Mr Abbott has previously raised concerns about gunman Man Haron Monis falling through the security net. "One of the things we are doing is asking ourselves do we need to go back and have a look at people who have come to our attention previously and see what is best done to ensure that the community is as safe as it humanly can be. This is the assurance I constantly give to the Australian people. "The first duty of government is the safety of the community."