It sounds like a made-up law.

But adverse possession, more commonly known as "squatting law", is very real — just ask the family of the late Henry Thompson Downie.

A decision in the NSW Supreme Court, revealed yesterday, handed Mr Downie's former home to a property developer who came across it in 1998, changed the locks and started renting it out.

The decision led to raised eyebrows across the country and raised many questions, the main one being: how can this happen in 2018?

"It is a very old doctrine of property law," University of NSW property law specialist Professor Cathy Sherry told the ABC.

UNSW professor and property law expert Cathy Sherry believes adverse possession is still needed ( ABC News )

"Essentially it means if you are in possession of land you can presume to have title to land."

The law, which dates back to 1900 in NSW, was originally aimed at communities with a large illiterate population.

"Like 200 years ago in England," Professor Sherry said. "[Back then] they were unlikely to have documents that they owned land."

In 2018 it is less common, but, as yesterday's story highlighted, it is still a thing.

How does it work?

The timeframe for how long a squatter must live on a property itself differs from state to state.

In South Australia the practice is illegal, but a squatter can apply for ownership if they can prove they have lived in the property for 15 years without the consent of the legal owner.

This came to a head in 2016 when squatter Iain Herridge was allowed to stay at an abandoned house in the Adelaide Hills when two people helped him pay off a $39,000 council rates debt, provided they gained an equity stake in it once he claimed rights to the property.

Squattor Iain Herridge pictured in the front yard of the abandoned home in 2016. ( ABC News: Carl Saville )

The 15-year timeframe is the same for Victoria. In Queensland, NSW and Western Australia the squatter must reside on the property for 12 years.

In the case of the $1.6 million property in Sydney, the property developer, Bill Gertos, made an application to the NSW Register-General under the NSW Real Property Act after renovating the home.

The family only found out last year after the police contacted them. They appealed to the Supreme Court but lost the case.

Mr Gertos' neighbours — whom he has never met — were yesterday calling for reform to the law, labelling it unfair.

No need for reform, 'it really is fair'

But according to Professor Sherry you could not run a property system without it.

"There will always be odd circumstances that arise," she said.

"For example, you could have 'great aunt Ida' who has lived in the same property for 50 years, but no-one really thinks about the fact that her dad technically left it in his will to her brother.

"This can happen, and in circumstances like that, it really is fair.

"And I don't think we'll see reform."