The man made it look like his recent work experience leaned heavily towards the research side of medicine, rather than seeing patients, they said.
They said his colleagues initially noticed he was falling short on the clinical aspects of his work but tried to help, thinking he was getting used to dealing directly with patients again.
However, they said they then realised large parts of his work were not up to standard.
RNZ understands the man saw about 70 to 80 patients in the six months he was there.
His job as a clinical researcher meant he did not diagnose them, but rather was given information by other doctors to follow up for research.
Te Whatu Ora Counties Manukau chief medical officer Andrew Connolly said the hospital was "deeply sorry".
It had reviewed the patients seen by the man and found there was no compromise to their care.
However, it had not yet spoken to them and was in the process of getting in touch, Connolly said.
The man started work at the clinic in February, before being stood down on August 1.
His contract was formally terminated on Wednesday and a police investigation launched.
-RNZ