Grey Power aged care spokesman Roy Reid said the trend raised serious concerns for elderly residents.
"When you have got a more settled staff and they are there long-term, the residents get to know and trust those staff, and that makes a big difference to their outlook on life," he said.
A high turnover of short-term migrants also made communication mishaps more likely.
"If there are language difficulties, that makes it very difficult, because an older, frail person is quite often a bit deaf anyway," he said.
The study by independent researchers Paul Callister and Juthika Badkar and Statistics NZ demographer Robert Didham used confidential Statistics NZ data which matches immigration, benefit and tax records.
It shows that the increased use of temporary migrants has been matched by fewer New Zealanders, especially those coming off benefits, obtaining rest-home jobs.
The number of people who had been on benefits in the previous three months fell from 26 per cent of people hired in 2001 to a low of 14 per cent in 2009, mainly because the number of beneficiaries also fell, and rose only slightly to 16 or 17 per cent in each of the past four years despite beneficiary numbers being higher in the recession.
Young workers under age 25 were a constant 17 per cent of people hired. Other New Zealanders increased from 56 per cent of new hires in 2001 to 62 per cent in 2009, but fell to 53 per cent last year as the number of temporary migrants hired increased.
Overall, people born overseas rose from 18 per cent of female rest-home caregivers in 2001 to 31 per cent last year, and 57 per cent in Auckland.
A third of the immigrants (32 per cent) were born in the Pacific Islands, 29 per cent in Asia (half in the Philippines), 18 per cent in Britain and Ireland, and 21 per cent elsewhere.
Aged Care Association chief executive Martin Taylor said rest homes were finding it "extremely difficult" to find suitable New Zealanders despite 134,000 people - 5.4 per cent of the workforce - being unemployed.
"When a facility advertises, it gets very few responses. Then it will go to Work and Income and get referrals, but the referrals are often unsuitable to look after the elderly," he said.
"We have to be very careful about the people we train and bring in, and someone who's had issues with the law, or poor working practices, or mental health issues, we have to be very careful about bringing them in.
"So people then turn to filling the gaps with immigrant workers. We find, by and large, immigrant workers are very good, very conscientious."
The association, the Nurses Organisation and the Service and Food Workers Union have all urged the Government to let caregivers qualify for permanent residence after a period working in New Zealand.
But an Immigration NZ spokeswoman said the agency had to ensure migrants did not displace suitable New Zealanders, and it was not considering "any policy change that would provide a pathway to permanent residence and citizenship for caregivers who come to New Zealand on temporary visas".
Annual renewal limbo 'stressful'
Eddie Budomo and his family are still living in fear on one-year visas nine years after coming to New Zealand.
Mr Budomo, 49, is an artist who trained and worked as a caregiver in the Philippines before coming to New Zealand initially on a visitor's visa in 2005. He found a job at St Andrew's Village in Glendowie within weeks.
He has done further training there and is now a medication and wound-competent healthcare assistant in the village's hospital wing, where his paintings hang on the walls. One depicts the hospital's much-loved cat, who died last year.
"I was an artist before I was a caregiver. I'm making use of it to make people happy, especially the residents," he said.
Mr Budomo's wife and two sons joined him in 2007. His wife Dolly now works in the kitchen at St Andrew's. Their elder son Jason, 28, who works in the maintenance team, married a Filipina caregiver at St Andrew's who has permanent residence, which has enabled Jason to get residency this year.
Their younger son Marc, almost 18, is on a student visa but will have to pay about $20,000 in fees as an international student when he starts training as a chef next year.
Eddie and Dolly Budomo still rely on Mr Budomo's work visa, which he has to renew every year. His current permit expires next June.
"It's really stressful and depressing," he said. "Every time your work permit expires you have this fear that you might not be able to renew again so you and your family will have to go home.
"It's a really difficult time."
St Andrew's human resources manager Lee Keegan said 47 of her 102 healthcare assistants were on temporary work visas. Last year 12 renewal applications were rejected at once, which would have forced the hospital to close. She had to lobby desperately to get the rejections overturned.
She said hospital-level care required qualified and empathetic caregivers and there were simply not enough suitable New Zealanders. Young local recruits were "shellshocked" by the clients' suffering and death, and local women who would once have been carers now had many other options.
She believes keeping families under a constant threat of eviction is "abusive".
