A teenager has admitted plotting to run down and behead a police officer during Anzac Day commemorations in Melbourne, in a case the prosecution alleged also included discussions of strapping a bomb to a kangaroo.

Sevdet Besim, 19, from Hallam in Melbourne's south-east, changed his plea to guilty in a brief hearing the Victorian Supreme Court over the charge of planning an act of terrorism.

He was arrested in counter-terrorism raids a week before the public holiday last year.

The offence carries a maximum sentence of life imprisonment.

The court had previously heard Besim had discussed his plans with a 15-year-old British teenager, explaining the date would make the attack more memorable.

The prosecution alleged the pair also spoke about a bizarre plan to paint a kangaroo with the Islamic State symbol, pack its pouch with C4 explosives and set it loose in the city.

The UK teen is serving a five-year jail term after he pleaded guilty to inciting the Anzac Day terrorist plot and other terrorism offences.

The boy was only 14 when he masterminded the plot to carry out a suicide attack on police officers at the parade from his bedroom in northern England.

Besim was a friend of Numan Haider, 18, who was shot by counter-terrorism officers after he attacked them with a knife outside the Endeavour Hills Police Station in September 2014.

The court heard Besim had been with Haider the day of his death.

Besim will return to court in August for a pre-sentence hearing.

Two others were also arrested in the raids in April last year.

Haran Causevic, 18, was given a good behaviour bond after pleading guilty to three weapons offences.

Mehran Azami pleaded guilty to more than a dozen charges relating to the importation of weapons after being accused of supplying weapons for the alleged plot.