One of two Sydney brothers accused of plotting to bring down an Etihad flight with a bomb hidden in a meat grinder was actually trying to prevent a terrorist act, a jury has been told.

Richard Pontella said contrary to what his client, Khaled Khayat, told police, he never took the bomb to Sydney international airport on July 15 in 2017.

Mr Pontella was opening his client's case on Tuesday at the NSW Supreme Court trial of Khayat, 51, and Mahmoud Khayat, 34, who have pleaded not guilty to conspiring - between January 20 and July 29 in 2017 - to do acts in preparation for, or planning, a terrorist act.

The Crown alleges the siblings, their older brother Tarek Khayat, who was involved with ISIS in Syria, and an unknown person referred to as "the controller" plotted the plane bomb attack and a lethal poisonous gas attack on people in a confined space.

Mr Pontello said Khaled Khayat did not enter into an agreement to do acts in preparation for a terrorist attack.

"The defence case is the polar opposite, " he said.

"He agreed to do those acts in order to prevent a terrorist act from ever occurring, which ultimately he did.

"The bomb never made it onto the plane. As far as he was concerned, it was never going to make it into the plane."

Prosecutor Lincoln Crowley QC has alleged that the bomb was in a meat grinder to be put into the luggage of another brother, Amer Khayat, who was flying out of Sydney on an Etihad flight.

Mr Pontello said his client told police he kept on refusing the controller's requests to get involved in the plot and had wanted to get his brother Tarek off his back.

"He was hoping Tarek would actually be killed in the conflict over there so the whole thing would go away and he would be left alone."

Mahmoud Khayat's barrister, Bruce Warmsley QC, told the jurors 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.

The Crown has alleged the bomb plot was abandoned after the travelling brother's luggage was found to be too heavy.

The trial is continuing before Justice Christine Adamson.