A Sydney real estate agency that used doctored photos to advertise a property where a man had lain dead for weeks is being investigated by NSW Fair Trading.

Key points: Initial photos of the property posted online made it look clean and tidy

Initial photos of the property posted online made it look clean and tidy They were changed after the ABC started making inquiries

They were changed after the ABC started making inquiries Its previous owner lay dead inside it for weeks before being discovered

At least two inspections have already been held at the unit, in leafy Wollstonecraft, which will go under the hammer on November 23 with a price guide of $775,000.

DiJones Neutral Bay advertised the property online late last month with several gleaming photos and a blurb describing it as "an unmissable opportunity".

The advertisement was amended after the ABC began making inquiries and contacted NSW Fair Trading.

A forensic analysis of the pictures revealed at least one of them, of the master bedroom, had been altered digitally.

The master bedroom looked presentable in the initial listing. ( Supplied )

But this is what it actually looked like. ( ABC News: Emma Elsworthy )

Neighbours described its previous owner as a person who lived a quiet life.

The ABC understands he died in the house in late 2017 but was discovered only when other residents noticed they had not seen him for months.

"He usually doesn't leave his light on and his light was on for a while," one neighbour said.

"We noticed a smell and flies hanging around, so we called the police and they came to investigate and they found his body inside."

This massive mark on the bedroom's floor could not be seen on the initial listing. ( ABC News: Emma Elsworthy )

This large stain was in the property's hallway. ( ABC News: Emma Elsworthy )

A spokesperson for the state's consumer watchdog, NSW Fair Trading, confirmed it had launched an investigation.

"NSW Fair Trading is assessing this matter further," they said.

The spokesperson said real estate agents "must act honestly, fairly and professionally with all parties involved in a property transaction".

"Failure to comply with the rules of conduct also attracts significant penalties."

CEO of REINSW Tim McKibbin described the photos as "unethical and illegal".

"It's my experience that it isn't a systemic problem — but like any other industry there are people within real estate who do the wrong thing," he said.

"We acknowledge that and we do wish they weren't part of the industry.

"It is clearly unethical and illegal, there's no doubt about either of those."

Large stains on the kitchen and cooker were not visible in the listing's original photos. ( Supplied )

But in reality, they were not hard to spot. ( ABC News: Emma Elsworthy )

The photos online have since been updated to more accurately reflect the house's condition.

The 86-square-metre flat was described as "a blank canvas ready to be reinvented" that "provides endless potential".

DiJones CEO Rob Ward confirmed in a statement the photographs were doctored but denied anyone from his business directed the changes be made.

"A third-party supplier engaged by DiJones Lower North Shore edited the published photos without consent," he said in a statement.

"We will of course cooperate and assist with any investigation that NSW Fair Trading may choose to undertake."

In a text message to prospective buyers on Thursday, it announced changes had been made to the online listing "to ensure that we accurately position and represent this property throughout the duration of the sales campaign".

Top Sydney property lawyer Victoria Holthouse said real estate agents should ask themselves: "Has the agent disclosed facts [about] the property which a purchaser could not find out from usual enquiries and inspections?"

This photo of the Wollstonecraft property's tidy living room was posted online last month. ( Supplied )

But this is what the living room actually looked like from a different angle. ( ABC News: Emma Elsworthy )

One neighbour said they saw the man only "half a dozen times" in 13 years.

"I started collecting his mail because it was stacking up in his letterbox," one neighbour said.

"I left it on his doorstep for him, but he didn't collect it.

"One day I got a knock on the door and two police officers asked me if I knew 'the deceased'."