The number of residential rental properties in Adelaide has risen 6% in the past 12 months but vacancies remain well below 2%, according to the Real Estate Institute of South Australia.

Inter-state investors have targeted Adelaide recently, lifting the number of properties available for renting. But the city's vacancy rate in September remained low at 1.5%.

One area targeted by investors has been postcode 5252, a region of the Adelaide Hills which includes a major mining project. The number of rental properties there has risen 152% in the past year, the REISA says.

The Hills precinct has again reported rental vacancies well below 1%.