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Mr McLean (63) was frail and suffered from schizophrenia and dementia.
He was living in a non-secure rehabilitation facility at Wakari and was last seen leaving the hospital grounds on October 12.
The SDHB has carried out an internal investigation into Mr McLean's care and how he managed to leave the hospital undetected.
In January it commissioned an external review, as per normal procedure.
''The draft external review has been received and it will now proceed through the organisational approval process,'' SDHB mental health general manager Louise Travers said.
Usually, a senior clinician from another DHB is asked to carry out such reviews.
''The mental health addiction and intellectual disability directorate leadership team will formally consider the review report early in November, before it goes to the SDHB executive for final approval,'' Ms Travers said.
Police and volunteers searched Wakari, Frasers Gully, Halfway Bush and Helensburgh for Mr McLean but found no sign of him.
Mr McLean was last seen wearing a dark polar fleece, trousers, a red polo shirt, brown slippers and a black puffer vest.