Auckland renters are sharing bedrooms or even "hot-bedding" to cope with sky-rocketing rents - with one apartment advertising a bunk in a bedroom complete with three room mates.

International students are believed to be the most vulnerable to exploitation in the rental market, with the cost to rent an apartment close to university campuses jumping out of reach for those on limited incomes.

The dire situation has been highlighted by a Facebook advertisement for a room mate to share a "master-room" with three other people.

Photographs of the central city abode show two sets of bunk beds in the bedroom, with two spare mattresses visible underneath one of the bunks.

Another photograph appears to show a bed in the living room of the apartment in The Wakefield, on Wakefield St.

"We are looking for a roommate in CBD area (close to the library, AUT, supermarket, stores) 130/w All bills included (power, water, internet). Share the masteroom with 3 ppl (4ppl in this masteroom with closet, balcony and own toilet) [sic]"

The woman advertising the bed told the Herald she was moving out of Auckland and needed to "find someone to replace me to get my bond back".

The apartment owner - a New Zealand woman - wanted four people living in the bedroom, she said..

The woman did not respond to further questions about the living conditions.

The Wakefield's building manager, a woman who identified herself as Bev, was unaware of the living situation when contacted yesterday.

After viewing the advert, she narrowed it down to one of two apartments - either a two-bed, two-bath, 105sq m pad, or a three-bed, three-bath, 186sq m unit.

She already had suspicions about the latter apartment, with a number of children and students thought to be living there, and planned to investigate this week, she said.

There were no rules limiting the number of people who could stay in the apartments, other than at the owner's specification. Most limited that to four people.

It's not the first time she'd come across unknown extra people living in the apartments, she said.

"They erect tents in the lounge ... and then somebody in the bedroom has a gas cooker and they'll be cooking their dinner in the bedroom, and it sets off the fire alarm."

Students' Associations president Rory McCourt said Auckland students were particularly vulnerable to over-crowding because rents in the city were so high.

"It's getting to really kind of fever pitch. We're at the absolute bottom end of the rental market, and that means we're the people likely to get exploited by these kinds of ads.

"Last year we saw instances of people advertising online offering one bedroom with someone already living in it, and sometimes it's been linked to sex ads online, where people can pay for their accommodation with their body."

Shannon Aitken, from Crockers property management, said he was aware of people building walls in their living rooms -- without consent -- to rent out the space. He said such situations were more commonly found with private owners who did not carry out regular inspections.

However, they would likely be in breach of fire regulations and could void the landlord's insurance cover.

"For fire, health and safety all the buildings have a certain number of tenants they can actually have residing in the building, so if you've got a one-bedroom with four or five people in it, the chances are you're putting the entire building in breach of its fire regs."

It was "a cultural thing" among some migrant communities, Mr Aitken said, but mostly it "comes down to the cost of renting".

He also revealed a trend known as "hot-bedding", where people -- often working opposite shifts -- took turns to sleep in a bed.

"So someone will be there during the night and someone will be sleeping during the day."

Apartment Specialists director Andrew Murray said while such over-crowding was likely illegal, it "happens all the time".

"It's very common. The most I've come across is a 48sq m two-bedroom [apartment] with 13 people living in it."

Most landlords specified the maximum number of tenants on the lease agreement, Mr Murray said.

Under the Housing Improvement Regulations 1947 a house is over-crowded if two people over the age of 10, of the opposite sex, are sharing a room, unless they're a couple. In 1994, the Ministry of Housing index classified a house as "seriously over-crowded" if there were more than three people per bedroom.