A judge did not properly instruct a jury during a complex case in which a man was convicted of plotting a terror act on a Sydney army base, a court has been told.

A lawyer for Saney Edow Aweys said Justice Betty King did not make "crystal clear" to the jury what they had to do in order to find his client guilty of conspiring to prepare for or plan a terrorist act in a 2010 trial.

Aweys, 28, was jailed for 18 years for planning to shoot as many soldiers as possible at Holsworthy Army Base to advance the cause of Islam, which he believed was under attack from the west.

Aweys and two co-offenders Wissam Mahmoud Fattal, 35, and Nayef El Sayed, 27, are appealing their conviction and sentence in the Victorian Court of Appeal.

Aweys' lawyer told the court Justice King did not appropriately instruct the jury.

"In the context of such a complex case the jury needed all the help it could get," the lawyer said.

The hearing continues.