Moving a dementia patient into a higher level of care isn't always the best approach, a prominent psychiatrist says, after a an elderly Canterbury woman was able to escape her rest home in someone else's car and drive an hour away.

The resident, in her late 70s, took a staff member's car from the Oaks Care Home earlier this month and ended up in Ashburton. She had earlier been assessed as having 'significant safety risks.'

Dr Matthew Croucher, Psychiatrist of Old Age at University of Otago, says some patients need interactions with healthier patients to maintain their mental health, and as a person's condition deteriorates and they need more care, the decision to move them to a new facility isn't always that clear cut.

"There are pros, as well as cons, of moving up to a higher level of care. Sometimes that can be very difficult to work out what's in the best interests of this person."

Dr Croucher said the level of care a dementia patient needs is in large assessed well - and it can't be blamed for incidents like that.

He said these incidents will occur because of the ageing population.

"There are more rest homes. There are more people in rest homes. There are more people with dementia living at home, and so of course there are more events like this."

He said the major concern is the inevitable number of patients that will enter the system, and the need to attract staff to deal with them.

"Older persons care is not considered to be a trendy which is a tragedy because it's really complex and really rewarding."

The rest home has promised an investigation will be carried out.