In the second incident, the man was reported missing from Seadrome Home and Hospital at 7.15pm on Monday. He was found stuck down a bank near a river.
Four different fire brigades and the police Eagle helicopter worked to get him out before high tide.
Mr King said the incidents were "appalling" and the DHB staff responsible for assessing the necessary standards of care needed to be investigated.
"Surely that's just a basic standard of care."
In the Blenheim case, it was worrying that the woman had been recently been assessed by Marlborough DHB as being fit for rest home level care, rather than specialist dementia care.
"When an incident like this happens you have to try and understand where the breakdown occurred, and if the person had only been assessed a few weeks beforehand then it reflects on the quality of the assessment," Mr King said.
"It might transpire that the assessor did everything correctly, but until a separate investigation's carried out you won't know."
Catherine Hall of Alzheimers New Zealand said it was important that a close eye was kept on dementia patients, as they could easily become confused and could have problems with mobility.
"From our perspective we would expect that rest homes always know where residents are and that they call for help very very quickly so that people don't get into trouble."
She said looking after dementia patients meant managing a difficult balance between giving residents personal autonomy and ensuring their safety.
"I think most of the rest homes around the country are very aware of the need for that balance and most of the time the services are of the quality that you would expect," Ms Hall said.
"But this is a service provided by people and sometimes things do go wrong."
Ministry of Health spokeswoman Emma Prestidge said patients with dementia who met the threshold for higher levels of care were required to live in a secure unit.
Ashwood Park Retirement Village manager Ross Bisset said last week the rest home followed all District Health Board instructions on the woman's care.
"She's free to come and go as she pleases."
The DHB did not immediately respond to a request for comment.
Associate Health Minister Peseta Sam Lotu-Iiga, who is responsible for aged care issues, also declined to comment.