Two men were arrested today during a counter-terrorism operation in Sydney, just weeks after five people were charged over a plot targeting an Australian government building.

Authorities, who are on high alert for potential lone-wolf attacks, gave few immediate details although the Australian Broadcasting Corporation reported that the home of one of those detained was raided earlier this month.

It cited police sources as saying hand written documents seized related to an alleged plot to attack a naval base.

"Police arrest two men as part of Operation Appleby," New South Wales police said in a brief statement, with a media conference due later in the day.

Appleby is a rolling operation investigating persons suspected of being involved in domestic acts of terrorism, foreign incursions into Syria and Iraq and the funding of terrorist organisations.

Earlier this month, two people, including a 15-year-old boy, were arrested and charged over a terror plot targeting a government building.

Three already in jail were also charged with the same offence -- conspiracy to conduct an act in preparation for a terrorist act.

Those charges stemmed from evidence gathered during pre-dawn raids in Australia late last year in which 15 people were taken into custody and an alleged plan to kidnap and behead a member of the public was uncovered.

Canberra lifted its terror threat alert to high last year, introduced new national security laws and has since conducted a string of counter-terrorism raids.

Six attacks in Australia have been foiled over the past year, according to the government, although several have not, including the terror-linked murder of police employee Curtis Cheng in October.