"That has the effect of bringing down average rent, even though the rents for houses and other bigger properties might stay the same."
Real estate data analyst Leon Hong said an increase in the number of landlords opting for short-term rental may have had a large impact in the active bonds received.
"Many home owners are doing short-term rental such as Airbnb, to increase the return on their investment properties," Hong said.
Letty Ho, head of property services at James Law Realty, said a 2-bedroom, which fetched about $400 a week in rent, could return between $3000 to $6000 if it was let out as a short-term daily rated rental.
A drop in new international student arrivals also contributed to a drop in demand for apartment rentals.
Ho said she took four weeks to rent out a near-new 2-bedroom apartment at $400 per week, and had virtually no enquiries for several weeks.
"If it was in January or February, it would have been rented out in a few days," she said.
Sales manager Judy Ang, who has been hunting for an apartment to rent for the last three months, eventually settled for just a furnished room in Parnell for $500 per week.
"There's not much options, and the good rentals are still way above what I am prepared to pay," she said.
Ang was looking at renting a 2-bedroom apartment with an asking price of $630 per week, but decided against it after working out that the 4-week bond, 1-week rent advance and letting fee would set her back close to $3900.
Renting a room is a stop-gap measure, she said, and a private arrangement with the landlord meant there was no written contract which gives her the flexibility of moving out with just a two-week notice should she find something more appropriate.
A property investor with five rental apartments in Auckland, who did not want to be named, said he too preferred "flexible arrangements" with his tenants.
His tenants are mainly international students or temporary migrants on work visas who enter flexible agreements which did not require them to pay any bonds.
"I treat them like short-term tenants because while it make sense to have them in the winter months, I will make more money from my properties if I used them as Airbnb in the summer," the landlord said.
"My tenants are happy because they have more money in the pocket, and I am happy because I have the flexibility to ask them to go with just a week's notice if I need my apartments."
Keith Ng said tenants and landlords who entered "grey market arrangements" would not be reflected in the tenancy data.
"However we're not seeing a significant drop in the number of properties available, so either these are new rentals rather than existing rentals, or they're still in small enough numbers that they're not making a dent in the overall market yet." Ng added.