The national rental index increased by just 0.6 per cent in September, the report found, the lowest monthly rise in rents since December.
The slowdown in rental growth "is a little surprising given rental vacancy rates remain so low and overseas migration is ramping up", the report said.
"A gradual slowdown in rental growth in the face of such low vacancy rates could be an early sign that renters are reaching an affordability ceiling," CoreLogic's Tim Lawless said, reflecting on the unexpected rental data trend in the report.
Lawless said high prices could now see tenants progressively seeking out living options in medium- and high-rise buildings, where monthly rents are typically cheaper.
Further evidence of a demand shift towards these higher density options was seen in the higher growth rate of unit rents over house rents, the report noted.
Capital city unit rents increased by 3.8 per cent over the September quarter, compared with 2.3 per cent in house rents.
Maximising the number of people living in a dwelling was another strategy renters were using to offset rising costs, Lawless said.