Prisoner fantasised about shooting judge

A prisoner accused of plotting to murder two South Australian judges fantasised about shooting one of them with a concealed shotgun at a cafe, an Adelaide court has heard.

Frederick Bernard Walkuski and fellow inmate Hendrik van Shaick have pleaded not guilty to conspiring to murder Judge Paul Rice and his wife, Judge Rosemary Davey, last year.

A third inmate, Wayne Buckingham, who thwarted their alleged plan by telling authorities, told the Supreme Court on Thursday that Walkuski knew where Judge Rice had coffee and wanted to walk up and "give him both barrels".

"It was at a table outside a coffee shop in an arcade," Buckingham said.

"He (Walkuski) said ... 'I'd like to get a trench coat, cut the hole out of the pocket, put my shotty inside the trench coat, walk up to him ... and just pull the coat open, pull the shotgun out and go bang, bang.

Walkuski, 64, allegedly offered 43-year-old van Shaick $20,000 to fire bomb the judges' home in retribution for a 17-year jail sentence imposed by Judge Rice.

The pair are also charged with conspiring to endanger the life of detective inspector Craig Wall by planning to fire bomb his car.

The trial continues.