Ibrahim Abbas is serving a maximum 24-year jail term after admitting to conspiring to plan an attack at Federation Square in 2016.

He asked Victoria's Court of Appeal to reduce his sentence, saying it was too harsh considering he admitted to his crimes and helped police.

Abbas gave evidence against his brother Hamza Abbas, cousin Abdullah Chaarani and friend Ahmed Mohamed, each convicted by a jury of the same charge over the same plot.

The Court of Appeal on Friday refused Abbas' appeal against his sentence.

"If anything, when one looks objectively at the horrifying nature of what was contemplated, and the sheer magnitude of the slaughter anticipated by the applicant in his fanatical zeal, the sentence imposed is to our mind quite moderate," three appeal judges said in their ruling.

They described Abbas' attitude as one of "unmitigated callousness and evil".

"He demonstrated no qualms, or pangs of conscience, about the tragedy and suffering he was about to unleash upon the community of which he was a member."

Abbas and his co-offenders were captured on CCTV scouting out CBD locations for the attack.

Abbas initially claimed they had gone into the city for ice cream, but later admitted it was a reconnaissance.

The group also bought machetes and built practice bombs before their arrest, three days before Christmas.

"That murderous attack was thwarted only by the resolute and dedicated efforts of law enforcement authorities, rather than by any hesitation, reluctance or repentance on (Abbas') part," the appeal judges said.

Australian Associated Press