Sister of murdered millionaire says she knows who killed him

But so far, no conviction has been made over the death of Michael Griffey.

The millionaire's sister Katrina told A Current Affair she first grew worried after he didn't return a call at about Christmas time in 2005.

Michael, 45, had been bludgeoned to death in his own garage - his body discovered by wife Diane and daughter Cassandra, up to four days after he was murdered.

Missing was his wallet, phone, two pages from his diary and cash from a safe.

The murder weapon was never found.

"Michael lay dead in a hot garage while they were partying on New Years Eve," Katrina said.

"I have cried, I've had so many tears about this, just the reaction."

The search for Michael's killer has always centred on the Griffey household.

Police first arrested Diane and Michael's son, Kenny, but he was released without charge.

Then, police charged Diane, who received a reported $1.5 million from Michael's life insurance policy. She pleaded not guilty.

But in a shock twist, the couple's daughter Cassandra confessed to her father's killing - but police didn't buy it.

Katrina told A Current Affair she believes she knows who carried out the murder.

"I just wish he was here, he shouldn't be dead," she said.

In court it was revealed that Diane told police her husband was covered in a black and white sheet, which police believe only the killer could have known.

The charges against Diane were later dropped due to lack of evidence.

The couple's son Kenny broke his silence last year, saying he was not responsible for his father's death.

He claimed a metal tool was missing from the garage after his dad's murder, and that his sister Cassandra gave police a pair of pants she was wearing at the time of Michael's death.

Kenny said the pants were covered in blood, but Cassandra had bleached them.

Michael was estranged from his wife and had relationships with several women. He had been dating Gillian Gould for five years, and she told a newspaper Michael was writing a new will and planning to divorce his wife.

A Current Affair and nine.com.au are not suggesting Diane, Cassandra, or Kenny are guilty - just that they were a focus of the police investigation at one point in time.

Meanwhile, Michael's murder remains unsolved.

"I just wish that they please come forward and just let Michael rest, and let me have peace of mind," Katrina said.

Diane did not comment when approached by A Current Affair.

Anybody with information about Michael Griffey's death should contact Crime Stoppers on 1800 333 000.