The informant was at the centre of a 1980s police operation in which he helped facilitate men with links to the Italian mafia grow cannabis on plantations near Bungendore, by holding out to them that they were under protection.

The operation was supervised by Mr Winchester. The men known as the Bungendore 11 were later arrested and charged over the Indian hemp plantations and due to face court for a committal hearing in February 1989.

After Mr Winchester's murder in January 1989, police approached the informant, who cannot legally be identified, and asked him where they needed to be looking.

In a transcript of his February 1989 interview with police tendered to court, the informant says he served the growers very well, by covering them up by saying the crop was Mr Winchester's project.

"Those are the people you have to look at," he told police.

Questioned by defence barrister George Georgiou SC on Tuesday, the witness refused to answer whether he had ever told anyone the crops were Mr Winchester's project, because it would incriminate him. When the judge Murray Kellam ordered him to answer, he replied that he had no recollection.

In the same 1989 interview with police, the informant said he never told people where his protection was coming from, but did say to a couple of police officers that he was working for the Australian Federal Police through Winchester and another officer Brian Lockwood. He also said he told the growers the protection was coming from police, but denied mentioning the name Winchester or Lockwood.

Under cross examination by prosecutor Michelle Campbell, the witness agreed he had many conversations with police after Mr Winchester's murder to help them with leads.

In response to the prosecutor's questions, he denied recalling conversations with police about various people who may be involved with the murder of Mr Winchester.

Asked if he knew who killed Mr Winchester the man replied: "Definitely not." He also denied recalling that he had told police to look into other police.

Prosecutors contend it was David Harold Eastman, 72, who shot Mr Winchester twice in the head on January 10, 1989, as he got out of his car in Deakin. Mr Eastman has pleaded not guilty. And the defence has pointed to other possibilities, including that the Italian mafia was responsible.

Also on Tuesday, the defence called its second witness Dr Helen Fraser, an expert in forensic transcription. The Crown has lead evidence of alleged confessions made by Mr Eastman and captured by police bugs in his flat.

However, the audio was indistinct, and so transcripts of the recordings were prepared by a Professor John French to help the jury with interpretation. Among the phrases the professor transcribed was “I had to kill him sitting down”.

Dr Fraser told the court the transcripts of David Eastman’s alleged utterances were not sufficiently reliable for the purpose of helping a jury in court. Dr Fraser said she used three things, the quality of linguistic cues, and internal and external context to evaluate the reliability.

She also gave evidence about priming, where context is suggested before or while hearing speech, and said it can be helpful in interpreting audio. But she warned that priming was a powerful force. “If we’re primed with misleading information, it can be used to hear something incorrectly,” she said.

In cross-examination, Dr Fraser agreed she held Professor French in high regard, and that she did not doubt his impartiality from the case nor his experience in preparing forensic transcriptions.

She agreed that when dealing with indistinct audio there was an advantage to having an expert prepare a transcript, and that there were words in Professor French's transcript that she could also perceive.

The trial continues.