Mr Bain was described as "penniless" by his lawyer, Michael Reed, QC, when the application for compensation was lodged in March 2010.
Most legal experts at the time thought Mr Bain would find it difficult to meet the criteria for a compensation payment.
Last week, Justice Minister Judith Collins said she had received the report of retired Canadian Supreme Court judge Ian Binnie and would meet him about it soon.
She said she would not release his report until the Cabinet decided whether to pay compensation
But the Herald has learned Justice Binnie believes that on the balance of probabilities, Mr Bain is innocent.
The judge came to New Zealand in July to interview Mr Bain and other people, including the police, whom the defence accused of planting a spectacle lens, a key piece of evidence in the prosecution case.
Mr Bain was convicted of murdering his parents, Margaret 50, and Robin 58, and his siblings, Arawa, 19, Laniet, 18, and Stephen, 14, in June 1994 by shooting them with a .22 rifle.
Former All Black Joe Karam has campaigned for years for Mr Bain.
He says Justice Binnie has undertaken an exhaustive investigation.
"I'm not at all surprised of course that Justice Binnie's found that David is innocent or innocent on balance of probabilities, if indeed that is what he has found because the evidence is overwhelming.''
Mr Karam says there's been a lot of speculation about the case, so he's looking forward to seeing the final report, when Cabinet releases it.
Mr Bain had served almost 13 years of his minimum 16-year life sentence when the Privy Council quashed his convictions in 2007 and ordered a retrial. He was acquitted after the three-month retrial in Christchurch in 2009.
Former Justice Minister Simon Power chose an overseas judge to examine Mr Bain's application for compensation because of the high-profile nature of the case in New Zealand.
The Cabinet faces a difficult decision over Justice Binnie's findings because of the polarised views about Mr Bain's innocence and because his case falls outside the 1998 guidelines for compensation for wrongful conviction and imprisonment.
Under those guidelines, eligible claimants must have had a pardon or had their convictions quashed on appeal without the order of a retrial.
But it has the discretion to pay compensation if it is convinced there are "extraordinary circumstances".
And there is a precedent for compensation in a case that falls outside the guidelines.
Last year, Aaron Farmer was paid $351,575 by the Government and received a statement of innocence and an apology for his conviction in 2005 for rape. He served two years and three months of an eight-year sentence.
The Court of Appeal quashed his conviction in 2007 and ordered a retrial, but the retrial never went ahead after DNA testing before it excluded Mr Farmer as being the offender.
Former New Zealand High Court judge Robert Fisher conducted the inquiry for the Government into that case.
For his 819 days in prison, Mr Farmer received $336,575 (or $410 a day) and an additional $15,000 for "improper conduct" by a police detective when interviewing him.
If a similar formula were applied to 13 years' imprisonment, the figure would be $1.94 million, with possible add-ons for pecuniary loss and improper conduct by the authorities.
The Ministry of Justice will report to the Cabinet on whether "extraordinary circumstances" exist, and if so, how much should be paid.
