A team of four private investigators is probing the 2010 killing of Feilding dairy farmer Scott Guy.
The team of former police detectives, led by Auckland-based Mike Crawford, had offered their expertise for free, the Sensible Sentencing Trust's Ruth Money told NZ Newswire.
The investigation came at the request of Mr Guy's widow Kylee Guy, she said.
The trust is co-ordinating the investigation, which will work in partnership with police.
Mr Guy was shot dead in July 2010 in the driveway of his Feilding home.
His brother-in-law Ewen Macdonald, who had helped run the family farm, was found not guilty of murder earlier this year.
Despite a nearly two-year police investigation failing to secure a conviction, Ms Money said more evidence might be unearthed.
The investigators will revisit the scene of the shooting and the farm, and speak to the family again.
"We have already had people come forward with interesting bits of information - which have to be verified. Some people are more comfortable speaking to a private investigator rather than police," Ms Money said.
She said the investigation would start from scratch and Macdonald would not be a prime suspect.
A law change in 2008 means the Court of Appeal can order a retrial if new and compelling evidence is discovered.
Macdonald was sentenced to five years' in prison when he appeared in the High Court at Palmerston North last week on six charges he pleaded guilty to early last year.
Macdonald admitted setting fire to the Guy family's old house in 2008 and vandalising Mr Guy's new home in 2009.
He also admitted poaching stags, killing 19 calves with a hammer, destroying 16,000 litres of milk and burning down a historic duck shooters' hut.