A “deeply radicalised” Islamic State supporter who helped a confidante source a gun used to murder NSW police accountant Curtis Cheng has been jailed for at least 21 years.

Mustafa Dirani, 26, was found guilty at his NSW Supreme Court retrial in March of conspiring with others in 2015 to do acts in preparation, or planning, for a terrorist act.

Mr Cheng was fatally shot in October 2015 by 15-year-old Farhad Jabar in Parramatta.

Justice Peter Johnson on Friday jailed Dirani, the fourth man to be sentenced over the crime, for 28 years with a non-parole period of 21 years.

The judge rejected a defence submission that Dirani travelled to various places in convoy with the confidante on the day of the shooting in order to have lunch.

-AAP