A previous landlord may have listed them out of spite for quitting, after rows about repairs or rents, or disputes with fellow tenants, and it can make it difficult for tenants to find somewhere to live, as was reported by ABC's 7:30 program.

But some tenants aren't even aware they're on a blacklist until they keep getting knocked back, says Ned Cutcher, senior policy officer of the Tenants Union of NSW. And then they could find it almost impossible to have their names removed from those lists.

"They're turned down when they apply to rent homes, and often have no idea why," Mr Cutcher says.

"As a result, they can be forced to consider options that would be considered sub-standard or go into arrangements like shared housing so their name is not on the tenancy, which may give them little stability."

The 2010 Residential Tenancy Act brought in regulatory control of the tenancy databases run by private companies and used by agents to screen prospective tenants. The act obliges real estate agents, for instance, to let tenants know, in writing, when they've been put on a database.

"But in the years since then, never once have I seen a written notice," says Mr Cutcher.

The act was also supposed to make it easier to get off the lists when the issues were resolved. But when Alicia Clifford had an argument with her flatmates, moved out of their shared house and had her name removed from the lease, she was startled to discover she'd been blacklisted after they'd been evicted months later.

"I was a student and didn't have much money and, because I couldn't get a rental anywhere else because of the blacklisting, I had to go back to live with my parents for years," says Ms Clifford, now 37 and working for the Land Titles Office. "It was devastating.

"I tried to resolve it with the real estate agent, but they weren't interested, and I thought about going to a tribunal, but it seemed so difficult and I didn't have the financial wherewithal to do it."

If tenants believe they've been unfairly listed, they can apply to the NSW Civil and Administrative Tribunal (NCAT) to be taken off.

An application to the tribunal usually costs $50, but applicants often have to pay a fee to each of the databases to find out if they are listed, and then have the complication, says the Tenants Union, of trying to work out who to take to the tribunal: the landlord, the real estate agent or the database?

An NCAT spokesperson said there were no statistics kept by the tribunal on how many matters of that type came before it. A spokesperson for NSW Fair Trading said people could come to them for advice, but there was no record kept of their numbers, either.

A major additional difficulty is that there's been a proliferation of rental blacklisting services throughout Australia that often operate quite secretly, with little transparency, and which often fed into other databases, Mr Cutcher says. "The databases often push the boundaries of what is allowed and, while they're sold as 'risk management tools', they're often quite punitive in the way they're used."

But the owner of one of the tenancy database companies, Gai Williams of Trading Reference Australia, says it's careful to look after tenants, as well as the owners and agents for whom they provide their service.

"I bought this business in 1995 and we abide by the legislation, but some of the other database agencies don't do the right thing and that gives us all a bad name," she says. "But we work closely with the Real Estate Institute and show due diligence to tenants and take their listing very, very seriously.

"We make sure to warn them and, if they do the right thing, they're taken off immediately. This is a person's life we're talking about and someone can do something wrong when they're young but then grow up, get married and have children and change. We're very aware of that."

Domain contacted TICA, Australia's largest tenancy database, but was told managing director Philip Nounnis was overseas and no one else from his office was prepared to comment.

The first national survey of renters' experiences published earlier this year found half of Australia's tenants live in fear of being blacklisted, which meant many do not complain about poor conditions or demand necessary repairs and maintenance on their homes. Over the past 20 years, the proportion of renters has increased from 25.7 per cent to 31 per cent.

Anthony Ziebell, who operates the database for tenants Don't Rent Me that warns about properties other tenants have said are undesirable, says blacklisted tenants are often quite unaware they have been listed.

"Someone may ask why their application was declined, but the agent says they were just unsuitable," he says. "They don't realise they have to ask the specific question, 'Am I blacklisted?'

"Then you can go to a tribunal to be taken off, but most people don't hold on to their old rental ledgers. I'm not anti-blacklist - we operate one - but we are pro-fairness and transparency and being able to be taken off if it's not right that someone's on one."