An Australian teenager who planned to run over and behead a police officer in an attack on an Australian Veterans' Day ceremony has been sentenced to ten years on prison.

Sevdet Ramadan Besim, 19, planned to attack police at the Anzac Day parade in Melbourne or the neighbouring city of Dandenong, on April 25, 2015.

Besim's scheme was uncovered when police in Britain discovered phone messages between Besim and a 15-year-old British boy.

Police said he was motivated by an extremist ideology and had expressed support for Isis.

Screenshot image taken from the 15-year-old British boy's phone that police said showed him plotting the Anzac Day attack (Greater Manchester Police)

In court documents, prosecutors said Besim and the British boy discussed packing a kangaroo with explosives and painting it with the Isis symbol before setting it loose on Australian police officers.

Victoria Supreme Court Justice Michael Croucher said Besim's decision to plead guilty was a major factor in his decision to impose a ten-year, rather than 15-year, sentence.

A wallpaper on the 15-year-old British boy's phone, discovered hidden under his mattress by police, showing Isis propaganda (Greater Manchester Police)

Mr Croucher said he had sought in his ruling to balance the need to protect the community from Besim's "evil" and "terrifying" plans against the young man's age, contrition and prospects of rehabilitation.

"I accept that Mr Besim's youth and immaturity are significant mitigating factors. He was just 18 at the time of the offending and is now only 19," Croucher said, according to a transcript of the proceedings.

The annual Anzac Day service commemorates the 1915 Gallipoli landings in Turkey, the first major military action fought by the Australian and New Zealand Army Corps during the FIrst World War. Hundreds of thousands of people attend the commemorative services around Australia.

Last year, a British court sentenced a 15-year-old boy from Blackburn to life in prison for his role in the plot.

ANZAC Day in pictures Show all 12 1 /12 ANZAC Day in pictures ANZAC Day in pictures Anzac-Day2-ap.jpg Australian visitors walk around during a ceremony at Tyne Cot Cemetery in Belgium - the world's largest British and Commonwealth war cemetery - to commemorate the Australian and New Zealand soldiers during the First World War AP ANZAC Day in pictures Anzac-Day7-gt.jpg A war veteran makes his way down Bathurst Street during the ANZAC Day parade in Sydney Getty Images ANZAC Day in pictures Anzac-Day18-gt.jpg A woman carries photos of a relative during the ANZAC Day parade in Sydney Getty Images ANZAC Day in pictures Anzac-Day1-gt.jpg Hundreds of Australians listen during the sunrise memorial service in Hellfire Pass in remembrance of all those who lost their lives on Anzac Day Getty Images ANZAC Day in pictures Anzac-Day4-afp.jpg Australians and New Zealanders attend a ceremony marking the 98th anniversary of Anzac Day at Anzac Cove AFP/Getty Images ANZAC Day in pictures Anzac-Day3-gt.jpg Former Australian POW Gordon Jamieson, 92, listens to speeches during the Wreath Laying Ceremony at the Kanchanaburi War Cemetery in Thailand Getty Images ANZAC Day in pictures Anzac-Day5-afp.jpg Australians and New Zealanders attend a ceremony marking the 98th anniversary of Anzac Day at Anzac Cove, in western Canakkale AFP/Getty Images ANZAC Day in pictures Anzac-Day9-gt.jpg Servicemen make their way down Bathurst Street during the ANZAC Day parade in Sydney Getty Images ANZAC Day in pictures Anzac-Day8-gt.jpg Crowds line George Street during the ANZAC Day parade in Sydney Getty Images ANZAC Day in pictures Anzac-Day6-afp.jpg The moon sets over the Australian War Memorial in the northern French city of Villers-Bretonneux AFP/Getty Images ANZAC Day in pictures Anzac-Day17-ap.jpg Children attend a dawn service ceremony at Buttes New British Cemetery in Belgium to commemorate the Australian and New Zealand soldiers who fought during World War I AP ANZAC Day in pictures Anzac-Day16-gt.jpg Archive footage is projected onto the walls of the Auckland Museum prior to ANZAC Day on 23 April Getty Images

Australia, a staunch US ally, has been on heightened alert for attacks by home-grown Islamist radicals since 2014 and authorities say they have thwarted a number of plots.

About 100 people have left Australia for Syria to fight alongside organisations such as Isis, Australia's Immigration Minister said this year.

There have been several "lone wolf" assaults in Australia, including a 2014 cafe siege in Sydney that left two hostages and the gunman dead. Also in 2014, police shot dead a Melbourne teenager after he stabbed two counter-terrorism officers.

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Besim's sentence may have seemed harsh just a handful of years ago, but must now been understood in view of recent mass casualty attacks in Europe and the United States, Greg Barton, a terrorism expert at Deakin University, told Reuters.

"It has to be seen in the context of realised and attempted low-fi, lone-wolf attacks in the last two years that are fairly unprecedented," Mr Barton added.

"Every month or two there's some development which makes the unimaginable suddenly very concrete."