The New Zealand Nurses' Organisation (NZNO) wants the government to come up with a fairer way of funding smaller church-run and charitable rest homes.
It comes as Presbyterian Support Southland announced plans to cut care by 157 hours a week at two rest homes.
A proposal to cut the staff hours at the two rest homes - Vickery Court in Invercargill and Rest Haven In Gore - were announced on Tuesday.
NZNO organiser Simone Montgomery said the proposed changes would reduce standards of patient care and put extra strain on nursing and care staff.
Ms Montgomery said the caregivers who worked in the homes would try to ensure no one missed out.
"But when you are a care-person down and you're already at a skeleton staffing level, something's got to break.
"That sometimes means that residents don't get all the care that they need in a day - and we call that care rationing. It means that sometimes when a resident needs to be a two-person hoist, that they're hoisted by one person and that's a risk to the resident and the care-person."
Earlier this year NZNO released the results of a survey it conducted with E Tū. It surveyed 1194 people working in aged care facilities and found three quarters of them either disagreed or strongly disagreed that staffing levels were sufficient to provide quality care for residents.
NZNO is also upset at out-of-date guidelines being used to justify the cuts.
Ms Montgomery said the 2005 Aged Care Staffing Guidelines were inadequate.
"The needs of residents have changed and the acuity's have increased exponentially to what was happening in 2005.
"When a facility says that they're only going to staff to just above those levels or at those levels we are always really nervous that someone's going to miss out."
Nurses have said the rationale for the staffing reductions was that DHB funding for aged care did not cover staff that would be considered "extra" when in relation to the 2005 guidelines.
Ms Montgomery said the proper solution should be increased funding to ensure standards of care.
"There could be a different funding formula for the likes of our religious and charitable facilities who have been in New Zealand for a very, very long time.
"It's really sad that this is happening and that these facilities are being put under so much pressure."
Presbyterian Support could not be reached for comment.