An associate of murdered Sydney man Michael McGurk has told the ABC that the property developer said he was going to blackmail the Sultan of Brunei.

British businessman Mark Burby went to the UK authorities when he heard Mr McGurk had been shot in front of his young son outside his home on Sydney's lower north shore.

Lawyers for the two men had been in touch recently about a lawsuit they were both involved in, to do with senior members of the royal family in Brunei.

"My lawyer just sends a very brief email; 'how you getting on, have you made the application?'" he said.

"Within four hours, five hours we get an email back, very chilling email, one line long saying 'Michael McGurk is murdered yesterday. Single gunshot to the head. Professional undertaking'.

"Even now I can feel goose bumps and a shiver down my spine when I hear that."

The two businessmen first met three years ago.

Mr Burby contacted Mr McGurk when he read in the media that the Sydneysider was suing the Sultan of Brunei for not paying for a rare, miniature Koran.

Mr Burby had also sued members of the royal family for reneging on a business deal and he wanted to see if the two lawsuits had any similarities.

At a meeting in the Channel Islands in 2006, Mr Burby claims the two men exchanged information about an ongoing private matter involving one of the Sultan's relatives.

"There's no short way around it, he was using that information, or was going to use that information, to blackmail the Sultan and this other senior member to say 'you will pay that Koran bill, you will resolve that Koran dispute or I'm going to put all this out'," Mr Burby said.

"The words he uses is 'I'll have it on the six o'clock news mate'."

Fearing for his life

There is no suggestion that any member of the royal family in Brunei is involved in Mr McGurk's death.

Mr Burby concedes the murder may be totally unrelated to him, but nevertheless he has feared for his life since hearing about it.

"Probably wandered around for a few hours, told my wife. I didn't tell her straight away, I thought how do I tell her this?" he said.

"I spoke to my lawyer, he was just about to go away on holiday and we found out this around lunch time our time, and within two or three hours it was we need to speak to the authorities about this."

He went to the local police near his home on the island of Jersey.

"Luckily the girl at the counter knew me, so you know it wasn't some nutter walking in off the street," he said.

"She got someone to speak to me. They sort of looked at me as if to say 'are you mad?'

"I showed them a few emails and documents and within 20 minutes thankfully, and to their credit, they took the situation very seriously and sent down some Special Branch people to see me, literally didn't want me to go home.

"They said just 'stay here'."

Mr Burby is now under police protection.

"I am very grateful for their response and how they've handled it and the way they're dealing with it at the moment and my wife feels a degree of comfort in the support they've given us," he said.

He has offered to assist police in New South Wales.

As well as the homicide investigation into Mr McGurk's death, the Independent Commission Against Corruption is looking into claims of bribery involving the New South Wales Labor Party.

There will also be a parliamentary inquiry into planning decisions linked to the murdered businessman.

Mr McGurk will be farewelled at a funeral at the Sacred Heart Church in Mosman this morning.