Terror experts have warned the danger of a "lone wolf" extremist driving a truck into Anzac Day crowds is high - as Isis urges its fanatics to launch an attack similar to the one on London's Westminster Bridge recently.

Police are taking extra precautions as thousands are expected to line the streets of Sydney, from Martin Place to Hyde Park, at dawn on April 25 to commemorate Australia's fallen soldiers.

As well as extra officers on duty, there will be several road closures in place as well as bollards in use to ensure there is limited access for vehicles to access the CBD.

Although authorities say there has been no specific threat about an attack on Anzac Day, Isis has been urging followers to carry out "lone wolf" attacks on commuters, queuing pensioners, children playing in the park and even market vendors, according to Daily Mail.

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The terror group's Rumiyah magazine used the chilling examples as it urged its followers around the world to target non-believers in a 38-page booklet published in different languages.

And after the horrific atrocities in Nice, Berlin, London and more - where extremists ploughed vehicles through helpless crowds of pedestrians - the threat of terror has never been higher, experts say, and police are taking no chances.

"In the last year or so, as that tactic has been used, we're more responsive to it - so we do look at vehicle mitigation strategies - that's really a fancy word for trying to block vehicle access to places of mass gatherings," NSW Police's Anzac Day operational commander, Superintendent Mark Walton, told the Daily Telegraph.

"Obviously the Anzac march route, the Cenotaph gathering, the Anzac memorial gathering fit that profile."

Police have remained tight-lipped on what specific measures will be put in place to prevent such an attack, but say they are well prepared and on high-alert.

Operation Callaway - a high-visibility policing strategy involving officers from a range of specialist units including riot squad and PolAir - will be in full force come Tuesday.

But Deakin University terrorism expert Dr Greg Barton told the Daily Telegraph there was little that could be done to prepare for a spontaneous attack.

"Of course, come 2017, we're now more than ever worried about lone wolf attacks - and that means some guy might wake up in the morning and literally decide he's going to drive his car into the crowd," Dr Barton told the newspaper.

On the eve of Anzac Day last year, police thwarted a 16-year-old boy's plans to commit a terror attack on a remembrance service in Sydney the next day.

The teen used an encrypted message app to attempt to obtain a gun and when that failed, a bomb manual.

He pleaded guilty to planning a terrorist attack last month and is awaiting sentencing.

The country's terrorism threat level remains at "probable".

Veterans travelling to Gallipoli for Anzac Day have also been warned that terrorists may target the commemorations this year.

The Australian federal government updated travel advice to Turkey after receiving information of a "high threat of a terrorist attack" targeting the event on April 25.

How vehicles became terrorists' favourite weapons

Terrorists using cars and lorries to run down people like in Westminster, Nice, Berlin and Stockholm "is becoming the standard of an attack", security experts warned.

Low-tech attacks using stolen and hired vehicles to randomly ram into crowds were becoming increasing common by terrorists unable to get hold of guns and explosives.

It is "nigh on impossible" for security services to monitor and stop potential terrorists planning such murders and the only way to prevent them is by using barriers to protect pedestrians.

There have been at least five major attacks using cars and trucks in the past nine months.

Stockholm

An Uzbek man allegedly killed four people when he drove a truck into a crowd in the centre of the Swedish capital on April 7.

Foreign Affairs Minister Abdulaziz Kamilov says the suspect, Rakhmat Akilov, had been recruited by the Islamic State group while in Sweden, where he had lived since 2014.

Police believe Rakhmat Akilov, 39, whose asylum application failed and was wanted for deportation at the time of the attack, hijacked a beer truck and drove it into a busy pedestrian street in the Swedish capital before crashing into a department store.

London

On 22 March, Khalid Masood, 52, drove a Hyundai 4x4 over Westminster Bridge, mowing down members of the public before crashing into a fence beside Big Ben.

He then got out of the car and attacked PC Keith Palmer with a knife.

Masood was shot twice by police and died when he was taken to hospital.

Five people died in the attack, including PC Palmer.

Antwerp

The day after the Westminster attack, a man in Antwerp drove into a crowded shopping area in the Belgian city in an attempted attack, but no one was injured.

Prosecutors today dropped terrorism charges against Mohamed R. as there was not enough evidence.

He remains in custody on a weapons offence related to the incident.

Jerusalem

On January 8, a Palestinian lorry driver mowed down and then reversed over Israeli soldiers, killing four and wounding 15 people, in a shocking copycat of the Berlin and Nice terror massacres.

Shocking video from the scene showed the driver reversing back over the soldiers, trapping 10 under his wheels, during the sickening attack.

Berlin

A copycat Isis-inspired massacre took place in Germany when a terrorist drove a truck through a Christmas market in Berlin.

Twelve people were killed in the attack on December 19.

Anis Amri stole a cargo truck and killed 12 people when he drove through a Christmas market next to the Kaiser Wilhelm Memorial Church at Breitscheidplatz.

Among the dozen people killed was the original driver of the truck, Lukasz Urban, who was found dead with a gunshot wound in the passenger seat.

Amri was a Tunisian who had failed to gain asylum status in the country.

He was killed in Milan four days later after an international manhunt.

Nice

On Bastille Day in the summer of last year, 86 people were killed when a truck was driven through crowds enjoying fireworks in Nice.

On the evening of July 14, 2016, a 19-tonne cargo truck was driven into crowds on the Promenade des Anglais.

Isis fanatic Mohamed Lahouaiej-Bouhlel was behind the wheel and after exchanging gunfire with police, the Tunisian migrant was shot dead.

Thousands had gathered on the seafront to watch the fireworks that night, 86 of them died and 434 were injured.