An Auckland landlord has found their house vandalised so badly, insurers have told them it's the worst damage they've ever seen.

The ceiling is completely ripped out, there's graffiti everywhere, and there's human faeces in the bedrooms.

The landlord says the worst part was knowing the damage was happening, but being unable to do anything to stop it until it was too late.

The large family home in south Auckland still looks respectable - from the outside. But as soon as you get to the front door, it's clear something very depraved has happened inside.

There's rubbish and smashed window glass all over every room in the house. The graffitied words "killers" and "die" are scrawled across the walls.

Joseph Yi's family owns this house, and with the help of their long-time property manager Murray Peat, are trying to understand how anyone could do this to them.

"I had goosebumps all over, and couldn't get much sleep in the evenings," says Mr Yi.

"All the ceilings have been pulled down, and even the ducting out of the main roofing has been pulled down," Mr Peat says.

Around every corner, there's a new horror.

"Some person has come in and put some human faeces there in the corner," Mr Peat says.

"The house was in a really nice condition. And when I saw this I thought, 'What kind of person does this?'"

The family asked their former tenant to leave just over a month ago, and it seems he abandoned the house.

The landlords found some of the damage around that time, but because their tenant still had the keys, it was illegal for them to change the locks for two weeks while the Tenancy Tribunal processed their request to take the house back.

While they were waiting, they hired security guards at night, but during the early morning hours the damage got worse.

Insurers say it's by far the most serious case of trashing they've ever seen, and repairs could cost up to $200,000.

Police are investigating, and the family desperately wants the culprits caught.

"I'd really like them to be rehabilitated, but I'd love them to come back to the house and help clean up, and do something positive to get a feel for how it is," Mr Peat says.

After a nightmare month, the landlords hope an alarm fitted to every room will mean the damage will finally stop.

Newshub.