He had Alzheimer's disease, and in the whole time he lived with her she had only one night away from him.

Sandra is among the many carers who hide what they feel is an unspeakable secret: they have wished their loved one were no longer living.

The first-ever study into the phenomenon has found high levels of distress among carers, with some actively contemplating killing the very person they are dedicating their life to.

Alzheimer's Australia says not enough is being done for the 70 per cent of people with the condition who live in the community and their carers, with a huge shortage of carers looming for people suffering from the common condition.

Study leader Siobhan O'Dwyer​ from Griffith University's Menzies Health Institute Queensland, said carers were under incredible pressure and often had no one to speak to about their feelings.

"I think this really reflects how trapped people are in this role, how isolated they are and the lack of support for these roles," she said.

The study, published in the journal Aging and Mental Health, found that almost 20 per cent of the carers interviewed had verbally or physically abused the person they were caring for, while the same number had wished their family member dead. Another two of the group's 21 participants had actively thought about how they might kill their loved one.

"None of them had actually shared these experiences or thoughts before they talked to us," Dr O'Dwyer said. "Most people tell us, particularly in the context of dementia, that once the person is diagnosed other family members drop away, friends don't stay involved… we need GPs and social workers and psychologists to be asking these questions and actually listening to the answers they are given."

She said while it was largely women in her study who reported thinking about the death of their loved one, the majority of murder-suicides committed among older people were committed by men.

"What we think that means is that women might report more thoughts, but men might be more likely to just go ahead and act," she said. "If we wait for these men to tell us it might be too late."

She said violence by carers typically occurred in response to a crisis, without the usual behaviour and thought patterns associated with domestic violence perpetrators.

Carol Bennett, the chief executive of Alzheimer's Australia, said the results were very concerning.

"We know there is a significant issue with elder abuse," she said. "Managing some of the symptoms of dementia can become very complex, particularly as the condition progresses."

Ms Bennett said organisations such as hers were overwhelmed with requests for support and resources in the community, with 1.2 million Australians already caring for someone with dementia.

"We know there is going to be a shortage of 150,000 paid and unpaid carers by 2029," she said.

Sandra said she can understand why people might begin to think about killing the person they were caring for.

While for many years she had lots of good days, it was towards the end, when her father refused to eat, that she began to feel like it would be better if he died.

"Even being a nurse, I didn't realise how hard it would be [on myself] for someone to just refuse to eat. He would just clamp his mouth shut and no amount of coaxing would work.

"It was pretty awful. I felt helpless, and it was almost like on some level he was trying his best not to be here."

Sandra says more in-home respite care that allows a person to be looked after without disturbing their routine is vital for carers.

"It's 24-7, and there is little relief," she said.

* name changed.