Justice Minister Amy Adams has conceded that the latest inquiry into whether David Bain should be paid compensation is taking longer than expected.

When retired Australian judge Ian Callinan, AC QC, was appointed to head the inquiry in March, Ms Adams said he expected to be able to report back within six months.

Mr Callinan - who is the second judge to investigate the compensation case - has now been considering the issue for eight months.

Ms Adams said through a spokesman yesterday that the inquiry was under way but had not been completed.

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"Mr Callinan's inquiry is taking longer than initially expected, but we're not concerned," the spokesman said.

The delay further deferred the conclusion of a drawn-out process which first began in 2011 and was expected to cost the taxpayer more than $1 million.

Mr Callinan's office said yesterday he would not comment on his report.

Ms Adams decided to start afresh on the compensation case in February, saying that the Cabinet did not have enough information to make a decision.

A 2012 report by former Canadian Supreme Court Judge Ian Binnie recommended compensation based on the view that Mr Bain was innocent "on the balance of probabilities".

Justice Minister Judith Collins then asked Robert Fisher, QC, to peer review the recommendation, and he found numerous errors in Mr Binnie's report.

If Mr Callinan came to the same conclusion as Mr Binnie, he has also been asked to say whether he believed Mr Bain's innocence had been proven beyond reasonable doubt. Cabinet would then consider his report before determining whether Mr Bain should get a payout.

Mr Bain stood to receive at least $2 million in compensation, not including pecuniary losses from his time in jail.

He was convicted of killing five family members in Dunedin in 1995, but was freed from prison after being found not guilty in a retrial in 2009.

Mr Bain's advocate Joe Karam said yesterday he had no comment to make.