Convicted terrorist Ahmed Mohamed has told a Melbourne court he only took part in plotting an attack on Melbourne's Federation Square because he was ignorant and thought Islamic State was cool.

Key points: A judge is considering what sentence to give three men convicted of plotting a terror attack on Federation Square

A judge is considering what sentence to give three men convicted of plotting a terror attack on Federation Square Hamza Abbas's lawyer said his client's offending was in a "lesser category" to his co-accused

Hamza Abbas's lawyer said his client's offending was in a "lesser category" to his co-accused Abdullah Chaarani's lawyer said his client had renounced any adherence to Islamic State ideology

In a surprise move, Mohamed, 27, took the stand at a plea hearing for himself and two co-offenders, Abdullah Chaarani, 29 and Hamza Abbas, 24.

Mohamed told the court he now hated Islamic State and only participated in the plan because he was too "chicken" to back out.

The two-day plea hearing is designed to assist Justice Christopher Beale in determining what sentence he will give each accused man.

During a lengthy trial last year, the jury heard the men had purchased machetes and metal pipes, as well as materials and chemicals to make explosives, and had tried to obtain guns.

They had also conducted reconnaissance in and around Federation Square, where the prosecution said they wanted to carry out an attack around Christmas in 2016 in order to maximise casualties.

On November 2 last year, after six days of deliberation, the jury found them guilty of terrorism offences.

In testimony at the plea hearing today, Mohamed said he did not grow up in a religious family but took to radical Islam when he came to Melbourne and wanted to fit in.

He is already serving a 22-year jail term for terrorism offences after he and Chaarani burned down a mosque in Fawkner a few weeks before his arrest in December 2016.

'I was captivated'

Mohamed said when he first started researching Islamic State and watching propaganda videos, he saw them "giving charity".

"As ignorant as I was, I saw them doing a lot of good. I was captivated," he said.

"I wanted to make bombs. I thought they [Islamic State] were cool."

He told the court he felt pressured by other Muslim boys when they talked about carrying out terrorist acts.

"I felt chicken to say I don't want to do it," he said.

Ahmed Mohamed, who was arrested in December 2016, told the court he "wanted to make bombs". ( ABC News, file photo )

He told the court that, after being in jail, he no longer supported Islamic State.

"I disagree with them completely. Their method is rubbish," he said.

He also pointed the finger at Ibrahim Abbas, the brother of co-offender Hamza Abbas.

Ibrahim Abbas was the key witness in the case, claiming to have been the group's ringleader, and was last year jailed for 24 years for his role in the plot.

"Every time we would meet we would talk about terrorism, he would say 'let's become martyrs'," Mohamed said.

"I would get scared."

The court heard Mohamed's son was born after he was jailed and is now two-and-a-half years old.

Defence lawyer John Kelly said Mohamed came to Australia from Egypt at the age of nine. He also spent time in Dubai as a teenager.

Terror plotter bullied in prison, court told

Earlier, the Supreme Court heard Hamza Abbas has been threatened and bullied in prison, as lawyers for the "childish and childlike" man sought sympathy from the sentencing judge.

During the 2018 trial, Ibrahim Abbas told the jury he had convinced his three co-accused to take part in the Islamic State-inspired plot.

Hamza Abbas's lawyer Felicity Gerry QC told the court her client had "intellectual limitations" and was a "follower" who was not aware "of the extent of what his brother was planning".

Ibrahim Abbas claimed to be the group's ringleader and gave evidence against the trio. ( ABC News, file image )

"He was not privy to the major arrangements of the major event," she told Justice Christopher Beale.

She also told the court Hamza Abbas was no longer radicalised.

But Justice Beale was unconvinced that the younger Abbas has turned his back on Islamic State ideology and questioned claims he could not "fully appreciate what the offending involved".

Ms Gerry argued her "childish and childlike" client was in a "lesser category" to his co-offender and that a long prison term would be more burdensome for him.

He had been "victimised, threatened and harassed all the time" in prison where he was isolated from other prisoners, she told the court.

Sentencing delayed by Fawkner mosque case

Abdullah Chaarani was obsessed with Islamic State and the events happening in Syria in 2015 and 2016, his lawyer Patrick Tehan QC told the court.

But Mr Tehan told the court Chaarani "categorically renounces any adherence to ISIS ideology".

Chaarani had an "intellectual, political and ideological" interest in the events in the Middle East at that time, in particular the clash between Sunni and Shiite Muslims, Mr Tehan said.

Chaarani is a Sunni Muslim.

The trio planned to carry out their attack at Federation Square at Christmas time in order to maximise casualties, their trial heard. ( ABC Melbourne: Simon Leo Brown, file image )

The sentencing of the men had been delayed because of the proceedings against Mohamed and Chaarani over the mosque attack.

During a plea hearing in that case, the men had made last-ditch attempts to convince the court they had renounced Islamic State.

Chaarani offered letters of apology to the mosque community and to Justice Andrew Tinney for an outburst in the court after the jury finding.

Chaarani and Mohamed are already serving sentences of 22 years' jail, with a minimum of 17 years, for burning down the Imam Ali Islamic Centre.