Wellington renters are being hit hard in the pocket, with prices hitting a record high average of $450 a week.
The figures, from Trade Me, show landlords hiked rents in the capital by an average of $40 last month.
That is higher than the average across the country, with the median asking rent rising by $15 from December to January.
While Auckland is still the most expensive place to rent a house, with an average price of $500 a week, rents in Wellington are increasing at the fastest rate.
Robert Whitaker from renters' rights group Wellington Renters United said it was unfair for landlords to make the most of the city's well-known shortage of rental housing.
"We think that landlords are taking advantage of the high demand at this time of year to hike up the prices when tenants are desperate for housing.
"It affects students, obviously, but it also affects non-student renters, and we've seen for a while now that the rental market is slowly aligning so that the majority of houses are being let at this time of year where landlords can demand the highest rents."
The organisation was started last year to help people campaign for better housing conditions, and Mr Whitaker said its membership was climbing.
He was particularly worried for the many families on a single income who could not afford to buy a home, and had rent soaking up most of their weekly budget.
It was frustrating that rents were going up without the quality of properties improving, he said.
"In Wellington, we have a lot of old houses with poor insulation and dampness and all those sorts of things.
"We're not paying more for more. We're paying more for the same because the demand is so high that every house is let and there aren't houses sitting empty because they're too poor quality for renters - renters need somewhere to live."
Trade Me Property head Nigel Jeffries said it was typical for landlords to adjust their rent in January, and Wellington was a classic example.
"All the uni students are coming back so supply is constricted and prices consequently head up, and also there is a quite a lot of tenancy churn [in] this bit of January."
Many students were wanting larger houses, and that was driving average rent prices up, Mr Jeffries said.
"The larger properties are up nearly 9.5 percent over the last year, up at $875, which is $100 more than Auckland.
"It's one of those examples where you get the Wellington market with a dearth of larger houses and the scarcity effect kicks in. A lot of student groups like the larger houses, but they're in short supply, hence the rents are heading up."
He said he did not expect prices to keep increasing monthly because they usually surged at the start of the year and then levelled off.