Sydney will install concrete barricades in a prominent pedestrian-only thoroughfare to mitigate against the risk of a vehicle attack by extremists, Australian police said on Friday, after recent incidents in Europe and the United States.

Australia's largest city was also considering more permanent measures such as bollards, garden beds or other landscaping features in Martin Place, which houses a war memorial, the headquarters of the Reserve Bank of Australia, the U.S. Consulate General, a hotel and many luxury retail outlets.

There was no specific threat to Sydney, police in New South Wales state said in a statement.

Earlier this month, a van ploughed into worshippers near a London mosque, the third such attack in the city since March. A car also crashed into pedestrians in New York City's bustling Times Square, killing one and injuring 22.

"A significant amount of work has been undertaken around vehicle mitigation strategies, including studies of world's best practice, which have been shared with all the relevant partners," the police spokesman said.

"Ultimately, the advice from police involves balancing the management of risk against the need for the public to enjoy access to public spaces," he said.

Sydney's city council said it was working with police to determine if additional bollards or barricades were required in other public places.

About 140 concrete bollards were erected in eight pedestrian locations around the centre of Melbourne, Australia's second-largest city, to reduce the risk of militant acts, the Australian Broadcasting Corporation reported.

Australia has been on a "high" national threat level since 2015, citing the likelihood of attacks by Australians radicalised in Iraq and Syria.

A staunch ally of the United States and its campaign against the Islamic State group in Syria and Iraq, Australia believes more than 100 of its citizens are fighting there.

Two hostages were killed during a 17-hour siege by a "lone wolf" gunman in a popular cafe in Martin Place in December 2014.

Five people were killed and scores injured after a 26-year old man drove a car into a crowd of pedestrians on a busy street in Melbourne in January this year. The incident was not terror-related, officials said.