I have looked through a Rotorua Daily Post article I wrote in January 2009 which really caught my eye, because it refers to the supply of houses in Rotorua to rent at that time was 230 - mainly due to the very slow sales market and houses being rented as the buyers were not buying.
The huge increase in house sales and prices since 2017, has tempted many "reluctant" landlords to quit, with the effect of an ever-decreasing number of available places to rent; that, coupled with big population growth, has created the current shortage and much higher rents.
Its called The Market, and The Market rules everything, from oversupply and low rents in 2009 and the absolute opposite 10 years later.
Critics can complain about property managers and greedy landlords as much as they like - anyone can throw mud - but instead of doing that, why not come along with ideas to turn the market around?
If you want to control the market, then go and live in the old Soviet Union, but if you want to influence the market, simply make being a landlord much easier and stop attacking them.