A Newshub investigation has revealed Auckland garages are being rented out illegally for more than $400 a week.

In a case that is currently before the Tenancy Tribunal, one property manager is accused of commandeering a garage, converting it and renting it out to separate tenants.

That's despite the garage apparently being included in the rental agreement for the tenant in the main house - something the property manager disputes.

The Tribunal has given the agent until August to provide evidence Auckland Council has approved the garage for residential use.

"If there is no Council approval for the sleepout to be used for residential purposes, then no rent may be claimed for its use as a residence."

Newshub's own investigation shows the former garage is consented only for use as a garage and can't legally be lived in.

Auckland Council says it has no record of any resource or building consent or any other permit to legally establish a housing unit or sleepout. There is no permit for anything "other than the establishment of the garage".

The sleepout was converted late in 2014 and has had several tenants since then, who have potentially paid thousands in illegal rent.

There was no proper separate power or water meter for the sleepout, meaning the tenant in the main house was billed for whatever residents in the garage used.

The Tribunal has already ordered the manager who let the garage, 'Debbie' Widharni Iskandar, to pay that money back.

Former tenant Jacine Greaves says tenants in the garage were "desperate, vulnerable people who had no other choice, people who had hit rock-bottom".

"She is a predator. That is my word to describe her, she is a predator."

Ms Iskandar says she manages more than 10 properties in the south Auckland suburb of Manurewa.

Newshub is aware at least three of those have garages that have been tenanted, yet according to Auckland Council records none of them is consented for human habitation and can't legally be lived in.

The prices have ranged from $250 to $560 a week.

That raises questions about whether those garage tenants past and present have also been charged thousands in illegal rent.

Ms Greaves, her partner and five children also rented a second illegal garage conversion at another property managed by Ms Iskandar. That cost $450 a week and had no bathroom or kitchen. The family shared facilities with multiple residents in the main house.

Ms Greaves believes Ms Iskandar's motivation is money.

"I think that is what her main focus is… to have her pockets lined with money. Who cares where the money comes from?"

As part of the Newshub investigation, a journalist posing as a potential tenant visited another of the Manurewa properties Ms Iskandar manages. She told him the garage conversion, which is illegal, was not available because it was already being rented out at $420 a week.