The decision to increase the alert level on Friday followed a spike in "chatter" detected in a sixfold increase in counter-terrorism operations during the past year.

Mr Abbott on Saturday repeated his assurance that the upgraded alert level did not mean a terror attack was imminent.

But it was designed to raise community awareness about the increased likelihood of a terrorist attack in Australia, with people urged to contact the authorities if they noticed anything suspicious.

"People should go about their lives normally, reassured that strong arrangements are in place to detect, to prevent and to respond to terrorism," Mr Abbott said in a statement on Saturday.

"The government is ... taking considered action so that you can continue to lead your lives confident that everything, everything possible is being done to protect you."

NRL and AFL finals in Perth, Townsville and Sydney are expected to undergo new security measures on Saturday. They are the second lot to be played under the high threat rating after two games in Melbourne and Sydney on Friday night.

Security will also be increased at airports, ports, military bases, government buildings and large public events.

NSW police intend focusing their efforts on popular landmarks such as the Sydney Harbour Bridge, and "soft targets" such as shopping centres.

Mr Abbott said the nation's security agencies always worked "quietly and purposefully" behind the scenes, and steps were being taken to make it harder for would-be terrorists to depart from Australia.

It's believed between 50 and 60 Australians are fighting with Islamic extremist groups in the Middle East, including the Islamic State of Iraq and the Levant and Jabhat al-Nusra.