Co-workers of a man who was fired for posing as a doctor at Middlemore Hospital raised concerns after they noticed his work was not up to standard.
Police are investigating after the man worked for six months in the respiratory clinic.
A source with connections to the respiratory team told RNZ the incident had been incredibly upsetting for them after a very busy two-and-a-half years on the frontlines of the Covid-19 pandemic.
They felt the deception had been sophisticated, they said.
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.
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.
University of Auckland has confirmed that the man, Yuvaraj Krishna, had previously been trespassed from its campus after posing as medical student for two years.
The university has confirmed he was trespassed in 2012.
The University of Sydney has confirmed Yuvaraj Krishnan graduated with a Bachelor of Science, in 2016.
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 1 August.
His contract was formally terminated on Wednesday and a police investigation launched.
Te Whatu Ora Health New Zealand said it was worrying a fake doctor was able to practise at Middlemore Hospital - but it was working on safeguards to prevent such situations in future.
Te Whatu Ora Counties Manukau confirmed he was employed in the respiratory unit from February until Wednesday of this week.
Health New Zealand said it supported the district's decision to refer the matter to the police.
A spokesperson said Te Whatu Ora had already been considering employment practices across the new entity, which took over from district health boards from 1 July.