The siblings plotted with their older brother, Tarek Khayat, who was involved with Islamic State in Syria, and "the controller", an unidentified person connected with the older brother, according to the prosecutor Lincoln Crowley QC.

Khaled Khayat, 51, was found guilty in May of conspiring, between mid-January and late-July 2017, to prepare or plan a terrorist act, but the NSW Supreme Court jury failed to agree on a verdict for his brother.

Mahmoud Khayat, 34, faced a retrial and another jury found him guilty of the same charge on Thursday afternoon.

Their motivation was said to have included supporting violent jihad and they were accused of doing many preparatory acts during the seven-month conspiracy.

The plane plot involved a bomb in a meat grinder being put into the luggage of a fourth brother, Amer Khayat, who was flying out of Sydney on an Etihad flight, Mr Crowley told the first trial.

But the plan was abandoned because the baggage was found to be overweight at the airport.

Khaled Khayat then proposed that he himself would arrange to take the bomb, Mr Crowley said.

"The controller told him not to do that because he had to stay for the continuation of the work here and had to find someone else," he said.

The second plot involved poisonous gas which the older brother was going to make at his home following instructions given by the controller.

When Khaled Khayat was arrested police found a piece of paper in his wallet that had Arabic words, numbers and symbols written on it.

The paper was examined by a forensic chemist and Arabic interpreters, who determined that one side of the paper included the correct chemical equation for poisonous gas, while the other side had further details relating to the gas.

In his three-day police interview, Khaled Khayat spoke of walking into the airport with the concealed bomb.

He said when he saw children at the airport he thought "Don't do it, don't be stupid, don't do it" and removed the bomb from the baggage.

But his barrister, Richard Pontella, told the jury that contrary to what his client told police, he never took the bomb to the airport and was actually trying to prevent a terrorist attack.

Mahmoud Khayat's barrister, Bruce Warmsley QC, said Khaled Khayat had admitted to police taking the bomb to the airport to put in Amer's luggage.

But his client told police he went to the airport with his two siblings to see one of them off.

"He was not aware one of his brothers was intending to murder the other brother by putting a bomb in his hand luggage," Mr Warmsley said.

Justice Christine Adamson will hear sentencing submissions on a later date.

A few hours before his brother was found guilty in Sydney, Amer Khayat was acquitted by Lebanon's military court, Reuters reports.

It reported Lebanon's state news agency NNA said Amer Khayat would leave Roumieh prison, where he has spent two years and two months, on Thursday night.

The military court also sentenced the three other Khayat brothers - Khaled, Mahmoud and Tareq - in absentia to hard labour for life, NNA said.

Australian Associated Press