Mr Pearson also said: "If patients have valuable items we always encourage them to leave them at home or alternatively give them to ward staff who will ensure the item is kept in a secure location."
Mrs King has been suffering ill health for a while and was in hospital for two weeks.
On the night before she was due to be discharged, a woman approached one of Mrs King's sons - who was visiting his mother - asking about her specially designed hearing aid.
Another son, Richard King, said the woman seemed to be very interested in their mother's hearing aid. "He actually thought she was a nurse at the time. She carried on about it for so long that he started to think it was odd."
The woman did not identify herself as a staff member, nor did her clothing make her appear to be a nurse.
The next day, as the family arrived to take their mother home, they realised her hearing aid had been removed.
Staff checked the entire room, bathroom and laundry in vain. "It was then my brother remembered the woman from the previous night."
Mr King - who gave up a job in Wellington to look after his mother full-time in Auckland - said knowing someone might have tricked them in such a way was heartbreaking - especially for their mother.
"These are her last days, when she's remembering and talking about things with us - sharing memories.
"But now it's hard for us to communicate because she can't hear and it's a shouting match and she gets agitated."
Asked what he'd like the person/s who stole their mother's hearing aid to know, Mr King said they only wanted the device back. "I want them to see the effects of their greed.
"Could they please have a thought for others when they're doing something like this. It's not just my mother involved - it's all of us too."
Police said investigators were waiting for CCTV footage to be provided by Middlemore.
Mr Pearson said the hospital was "very sympathetic to Mrs King's loss" and an investigation had "concluded the hearing aid had been misplaced rather than stolen".
"Ward staff made every effort to make Mrs King's stay as comfortable as possible ... Middlemore Hospital takes security issues very seriously."
- Additional reporting: Morgan Tait