"People are choosing to rent, rather than buy, because paying rent is cheaper than having a mortgage," she said.
Some people had moved to Wanganui for work, but were only planning to stay for two or three years and did not want to enter the housing market.
There were also many retired couples who went travelling and rented out their houses, she said.
In some parts of Wanganui, such as Castlecliff, rent had decreased as there was an over-supply of properties.
Ms Onishenko expected average rents to continue to rise as more high-end properties became available for rent.
Nationally, the average rent for a three-bedroom house was up $15 year-on-year to $379 per week. A three-bedroom flat was up $11 to $381, while three-bedroom apartments were down by $11 to $527.
Apartments were also the most expensive mean property type for four bedrooms, up $84 to $669 per week, compared to a $52 increase to $490 for a house and dollar increase to $470 for a flat.
Tenants Protection Association manager Helen Gatonyi said she wasn't surprised by the increases, as it was common for the association to hear of tenants whose rents had increased by $20 or $30, or in some cases, by $100 or more.