Australian prime minister Tony Abbott said there had been a "heightened level of terror chatter" since the siege at a Sydney cafe last week, BBC reported.
According to Abbott, the National Security Committee had met to discuss the development on Tuesday. However, the terrorist threat would remain at "high" and would not be escalated to "extreme," he added.
Earlier, memorial services were held for two hostages who fell to a gunman's bullets in a siege.
A 16-hour stand-off at the Lindt café in Martin Place ended with the gunman being shot dead by commandos, but Tori Johnson and Katrina Dawson died in the stand-off.
The funeral service for Johnson, the manager of the cafe, at St Stephen's Uniting Church was attended by several of the 17 other hostages, along with New South Wales Premier Mike Baird.
Over 1,000 people also attended a separate memorial service for Dawson at the University of Sydney, where the barrister was a student.
At Tuesday's meeting of the National Security Committee, Abbott said he had received a briefing from security agencies of "a heightened level of chatter amongst people who we would normally think of as terrorist sympathisers", The Telegraph reported.
"We don't know when and how an attack may come, but we do know there are people with the intent and the capability to carry out further attacks."
"I am alerting people to the fact that the terror threat remains high and as you all understand, at this level, an attack is likely," he said.
"That's why it's important that people remain alert and aware as well as reassured that our police and security agencies are doing everything they humanly can to keep us safe."
At high, a terrorist attack is seen as likely while an extreme alert level meant an attack was imminent or had occurred.
The Sydney siege had not been officially branded by Australia as a terrorist attack.
Abbott added that the government would not speculate about any specific future attacks, and emphasised only that "there has been a heightened level of chatter amongst people who we would normally think of as terrorist sympathisers".