Dunedin property spokeswoman Liz Nidd said a substantial number of leases for flats in Dunedin were full 12 month leases, despite not being occupied all of that time.
"Those properties sit there empty over the summer in most cases," she said.
Many students continue to pay rent after leaving town at the end of the academic year, she said.
"As a property owner, if you are going to provide property for that university market you've otherwise got an asset that's going to sit there for three months non-earning."
The rental property market had "always been like that" and students competed over flats for the next study year as early as August, she said.
"Those flat lists go out in August ... you've got people signing up in August to make sure they've got that special flat that they want for the next year.
Most landlords hoped to find tenants during the initial student market sales period and those that missed out could find themselves short.
"Those owners are sometimes looking at not getting the full year for the next year because if they don't get them leased before the students head off for Christmas there's a good chance they're not going to get them sorted before the students come back."
While demand for flats had been constant over the years, Ms Nidd said boundaries for student accommodation had changed, resulting in a "bit of a moving feast".
"The boundaries of the campus have changed and there's quite a lot of multi-unit developments put in right in the heart of campus."
The university itself was also offering more accommodation, she said.
Rents in January in Dunedin showed an average of $361/week, which dropped to $294/week in June, then jumped back up to $390/week in October - perhaps picking up on students securing properties for the following year.
It's a big change from 1993, where rent in the student town was an average of $145/week
Auckland's rising house prices push rent up
While many have chosen to rent rather than buy in Auckland due to rising house prices, average rent in New Zealand's biggest city passed the $500 mark for the first time this year.
Rental bond information from Tenancy Services, mapped into interactive graphics by the Herald Insights, showed the city's average rent now stands at $514/week, rising from $491/week in January. North Shore rents are the country's highest at $543/week and Auckland's rents have been rising strongly since 1993, the information showed.
In 1993, Auckland's average rent sat at $200/week, with the North Shore a little higher at $209/week.
With the city's rents well above the national average, the rental gap between Auckland and the rest of the country is widening.
Comparatively, rents have an average of $414/week in Wellington, $394/week in Christchurch and $390/week in Dunedin.
In Christchurch, however, rents rose steeply after the earthquakes in 2010 and 2011.
While average rent in the city was $291/week in August 2010, it's risen to a current figure of $394/week and peaked at $434/week in February this year.
The data also showed that rents in Wellington and Dunedin fluctuate on a seasonal basis, due largely to student populations.