One of the targets: The men were allegedly planning to bomb an Etihad plane.

Two brothers accused of planning an IS-inspired terrorist attack in Sydney plotted to put a bomb on a plane at Sydney Airport and separately planned to release lethal gas, a jury has been told.

Mahmoud Khayat, 34, and Khaled Mahmoud Khayat, 51, have pleaded not guilty to one charge each of conspiring to do an act in preparation/planning for a terrorist act, after they allegedly planned the acts from January 20, 2017 until their arrest on July 29, 2017.

On Monday, at the opening of a trial in the NSW Supreme Court, Crown prosecutor Lincoln Crowley, QC, said the conspiracy was "formed and developed" when the men used apps including Telegram to contact an older brother fighting alongside Islamic State in Syria, and another person called "The Controller" who has not been identified.

Mr Crowley said The Controller sent instructions on how to make the bomb and lethal gas, suggested possible places to deploy the gas, and gave tips about timing and weather conditions.