A former top cop, and Teina Pora's private investigator, says more can and should be done to convict Malcolm Rewa.

The New Zealand Police Force has admitted for the first time that Teina Pora is innocent of the 1992 rape and murder of Susan Burdett, after Pora spent more than 21 years in jail.

Commissioner Mike Bush says he believes that Malcolm Rewa is the likely killer, but says Rewa cannot face another trial because there's no new evidence to put before the courts.

Former police detective, now private investigator, Tim McKinnel told Mike Hosking the Police admission is a step in the right direction.

But he believes more can be done.

"They could start with a full and objective and independent review of the case, but I don't think there's any doubt that the criminal justice system and the Police investigation categorically failed, and we haven't really had that addressed yet."

"The last seven years in particular, the focus has very much been on collecting more evidence against Teina Pora, and clearly given developments yesterday that's no longer tenable, and so the question remains what work has been done to try and solve Susan's murder."

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