Dr Bonning said elderly patients, most frail but without physical disabilities, were being abandoned at hospitals because families had not properly prepared for their care in their absence.
"You will get a small spike of it before a long weekend ... and before and around holidays which indicates the family is trying to look after them during office hours but then have their lives to get on with," he said.
"It certainly is happening ... but I think we have got a bit of a duty to look after our elderly population."
Waitemata District Health Board communications manager Errol Kiong said "granny dumping" was "apparently a phenomenon at every DHB around the country".
Dr Bonning blamed a lack of planning for a problem that he said emergency staff did not have resources to deal with.
But hospital staff had a duty of care to look after the elderly and could not turn them away, so many were staying for at least 16 hours in observation units.
He said that because the elderly population was expected to rise, the problem would probably become worse.
"I do want to suggest that people plan for these eventualities because we do find it very frustrating when people come in and you look at them and you think 'why wasn't this sorted out when it could have been so much simpler'?" he said.
"It's not in everyone's family relationship to be able to be at the beck and call of their elderly relatives, but if you're not going to do that, think of some alternatives when there's lots of infrastructure around.
"Plan it, as opposed to being surprised at the last minute when the straw does break the camel's back."
Lawyer Roger Laybourn said the law on dumping able-bodied elderly people at hospitals was unclear and he couldn't remember any cases in which someone faced criminal charges for doing so.
He said most neglect cases related to children under the age of 16, but the incidence of cases involving the elderly could be an issue that should be looked at more closely.
"You wouldn't establish there was breaking of the law without knowing the individual circumstances, and you would have to know that there was a legal obligation to care," he said.
"That would have to be the beginning of any potential issue of breaking the law and that would arise only from knowing the relationships and the obligations involved."
Aged Care Association chief executive Martin Taylor said a lack of readily available short-term respite care for the elderly was a problem for some families who were turning to hospitals when they couldn't find an alternative.
"What we more frequently have is people ringing our office saying 'how do I get respite care for mum and dad'?" he said.
"They could get to the situation where the lack of respite care means that they then think the best place we can go is the hospital."
Mr Taylor said the daily rate respite care facilities received for short-term patients was not good and they preferred to have long-term residents.
"What's been happening is facilities are saying 'I can't afford to keep two beds aside for respite care and I will take respite care only if I have an empty bed because I am waiting for a long term resident'.
"The result of that is the supply of respite care beds or planned beds is minimal."
He said he struggled with the notion that families would abandontheir elderly relatives during holiday periods.
"I know it happens but it is very disappointing - it's not the way to treat the elderly.
"On the other side of it, if the health system isn't looking at the causes of that problem and seeing if there is a way of addressing them, then that's equally poor."
