That's the explosive claim in a new book The Man Who Knew Too Much by Dr John Jiggens.

Dr Jiggens spoke to a witness who had been "chased away by the police".

"He was able to give me a detailed account of the murder," Dr Jiggens said.

In his previous book The Killer Cop and the Murder of Don Mackay Dr Jiggens pointed the finger at banker Frank Nugan and former detective Fred Krahe.

"On the how of the murder, I went for a two assassin theory, which is wrong there were seven as it turns out," he said.

"It was a much bigger operation than I thought."

Dr Jiggens said the book outlined information Don Mackay knew about an international drug smuggling operation.

After researching cannabis prohibition in Australia as part of his PhD, Dr Jiggens started to have doubts about the theory that James Bazley killed Mackay.

"I began to have doubts and I began to believe he had been framed. I began to believe Frank Nugan was involved in the murder," Dr Jiggens said.

Dr Jiggens spoke to the Mackay family lawyer, Ian Salmon, who alerted police that Mackay was missing and discovered his mini van in the car park at the Griffith Hotel.

"I interviewed Don Mackay's solicitor Ian Salmon," Dr Jiggens said.

"He had worked with Barbara Mackay and concerned citizens of Griffith who were trying to get justice for Don."

Dr Jiggens said Mr Salmon told him he believed there had to be more than one person involved in the murder.

"He convinced me there had to be two or more assassins," he said.

Dr Jiggens said Mr Salmon discovered the murder scene, but found no evidence of anyone being dragged.

This made him believe that more than one person was involved, because Mackay was 190 centimetres tall and weighed 93 kilograms.

Dr Jiggens said he had researched this version of events.

"It's a very well organised murder and that's what you'd expect when you go to the carpark of the Griffith Hotel. It was 6.30pm on a Friday night - it would be really difficult circumstances to commit a murder. There would be people all around and for one person to do it is extraordinary. It's the sort of operation that needs a big team."

Dr Jiggens said that sort of operation was exactly how he would expect a former policeman, like Krahe, to run it.

In 2013, Dr Jiggens spoke to the witness who told him his version of events.

He said the witness believed there were police in the NSW Police Force who were protecting Krahe.

"Because you have ex police involved, some of the police who are still in the police force are covering up for them and because of that, people don't want to go to NSW Police because they could end up dead," Dr Jiggens said.

When asked if he had ever had anyone take issue with the content of his first book, Dr Jiggens said "obviously people have disagreed with me (but) no one has physically threatened me".

Dr Jiggens said he had tried twice, unsuccessfully, to access witness statements about Mackay's murder.

He said he had previously tried to access another file that had disappeared.

"I have a great fear that the witness statements will disappear," Dr Jiggens said.

"The police are denying me the witness statements so I conclude there's something quite important in the witness statements. That's why I figure they'll disappear."

Dr Jiggens will launch the book at Parliament House on October 14.

He has invited Premier Mike Baird (who has the authority to release the witness statements) to attend.