Lawyers demanding payment from drivers who overstay time limits in shopping centre car parks are operating in a grey area that is unlikely to be upheld in court, a legal expert has said.

Drivers in Queensland and Victoria have told the ABC that demands for more than $70 are regularly placed on vehicles in car parks with no clear signage on time limits, or before the set time limit was up.

Others said they had been pursued by lawyers demanding payment and threatening court action.

A woman in Toowoomba, west of Brisbane, said she was pursued for money by a car park operator because her ticket was displayed "upside down".

Carparking companies argued the system was crucial to prevent drivers staying too long on private land, but a legal expert said payment demands were often too high when measured against the infringement.

Steve Spalding from motoring lobby group the RACQ said it supported the penalty system if terms and conditions were made clear to drivers.

"If you didn't comply with the parking conditions, you should expect to have to be bound by those penalties," he said.

"Motorists need to be absolutely clear on the conditions of parking in that location.

"Effectively a person is using someone else's property. If someone parked their car on your front lawn you would get fairly agro about that unless you had given them permission to do that."

'Lawyers are making empty threats'

Victorian resident Martin Gleeson received dozens of infringement notices from Australian National Carparks (ANCP), which previously managed a car park for Woolworths at Preston, north of Melbourne.

After years of ignoring consistent warning letters, he said he had not paid them a cent.

"Unfortunately, most people cave to empty threats from solicitors and debt collectors because they just don't know any better," he said.

In a letter to ANCP lawyers, Mr Gleeson argued he was the owner of the car, but that there was no proof he was the driver.

"I am under no obligation to assist you in your slimy attempts to trick people out of their money," he wrote.

In Toowoomba, several people have complained about a shopping centre car park also controlled by ANCP.

Toowoomba residents have reported finding $88 infringement notices on their cars, with some saying they found the notices upon returning to their cars before the allocated time limit.

A ticket notice left on a car at Inala Plaza Shopping Centre in Brisbane's south-west. ( Supplied )

Toowoomba resident Annemarie Stower said she believed she had to pay up, after she accidentally displayed her parking ticket upside down on her dashboard.

"My ticket was up the wrong way but I was under the hour — it was free time," she said.

"I ended up sending a message to them [ANCP] explaining, but they still said we had to pay it."

ANCP has not responded to the ABC's requests for comment.

Another Queensland driver told the ABC he received a $77 ticket from Traffic Monitoring Services (TMS) last month, which manages a car park at the Big Top Shopping Centre at Maroochydore on the Sunshine Coast.

When the man refused to pay, lawyers for TMS sent a letter demanding $99 and threatening legal action if the ticket was not paid.

Industry body Parking Australia CEO Lorraine Duffy defended the actions of TMS, saying car park overstayers cost the retail industry up to $15 billion a year.

"There is a need to regulate a turnover of vehicles and prevent those who are using facilities designed for a specific purpose in contravention," she said.

Who are the parking companies?

A sign commonly used at Secure Parking sites that set the rules of the car park. ( ABC News )

Companies issuing notices in Queensland include CelloPark, Australian National Carparks (or Prestige Parking), PriPark and TMS, which collects money for large car-parking firm Secure Parking.

Of these, all but Prestige Parking are members of industry body Parking Australia, which runs an accreditation scheme to "stamp out rogue operators".

The ABC understands Prestige Parking changed its name from Australian National Car Parks, which faced court action in New South Wales in 2013 over alleged harassment.

In letters to customers obtained by the ABC, Woolworths previously defended its decision to use the company in "very few" of its car parks that were subject to "heavy abuse by commuters and local workers".

But a Woolworths spokesperson said the supermarket chain cut ties with the company at its Preston store more than six months ago.

Aldi, which has also used Prestige Parking, declined to comment.

In most cases, the companies are legally able to obtain addresses to contact people they are chasing.

ParkPay said it would contact people who had an "outstanding debt".

Can a private company issue an infringement notice?

Queensland property lawyer Tim O'Dwyer said it was quite legal to issue a notice, based on the law of contracts.

He said car park companies argued anyone entering a car park had agreed to a contract [written in fine print on the car park sign] to stay for a certain amount of time or face a penalty.

"When you see the sign and you keep driving you are accepting the terms of that offer and you then create a contract between you and the car park operators," he said.

But a Queensland Government website quoting Australian Consumer Law said car park rules could be considered unfair by a court if they "suggest a consumer who enters your car park has automatically agreed to your terms and conditions".

Payment can be demanded from overstayers, but only government bodies and courts can issue a demand and call it a fine.

Lawyers say it is quite legal to issue a notice, based on the law of contracts. ( Supplied )

What happens if you don't pay?

Several people have shown the ABC letters sent to them by lawyers threatening legal action and demanding customers pay a bigger fine for ignoring the notices.

A follow-up letter sent by TMS to drivers who do not pay a parking fine straight away. ( Supplied )

But Mr O'Dwyer said such demands were unlikely to end up in court.

"I'm not aware of anyone actually being sued by the parking company or the shopping centre," he said.

"I suspect they're probably working on the basis that people will be spooked and won't do it again."

Are privately issued breach notices enforceable?

This is not a question that can be easily answered.

Mr O'Dwyer puts it like this: "If you don't pay, they might huff and puff and they might write you a letter or two but you don't have to pay until a court says you have to."

Parking management companies would need to take the car owner to the Queensland Civil and Administrative Tribunal (QCAT) to prove they had breached a car park's contract.

Mr O'Dwyer said that would be very difficult to prove.

A spokesperson for Queensland's Office of Fair Trading (OFT) said in a statement although it did not specifically regulate private car parking, ignoring a payment notice was not recommended.

"Consumers who receive a payment notice, and wish to dispute it, should contact the private car park operator to state their intention not to pay, giving reasons," the OFT spokesperson said.

"If the matter remains unresolved, consumers should consider lodging a claim in the Queensland Civil and Administrative Tribunal (QCAT) to seek relief from payment, or seek independent legal advice."

The OFT spokesperson also said private car park contract terms and conditions should be fair, transparent, and in plain English.

"If a consumer encounters misleading private car park signage — for example, a sign states the maximum cost is $200 and they are charged $250 — the consumer should make a complaint with the OFT," the spokesperson said.