An investigation by a NSW Supreme Court-appointed special purpose liquidator into Ambient Advertising is examining allegations the firm may have routinely and deliberately overcharged some of its clients by overstating the scale of advertising campaigns.

The investigation is causing significant ripples in the media and advertising industry, as, prior to its collapse, Ambient was a market leader in providing "below the line" advertising, which includes street poster advertising and experiential campaigns.

In July, the liquidator Nicholas Crouch contacted around 70 Australian advertising and media agencies to ask if they had ever directed Ambient to provide less advertising than ordered and paid for by clients.

Mr Crouch is attempting to determine if there was a deliberate fraud and, if so, who was responsible for it, in findings that could be referred to regulatory or law enforcement agencies.

"We have undertaken an investigation into the advertising contracts. In 80 per cent of cases tested in a sample of contracts, there is a discrepancy that suggests that clients have not been given what they paid for," Mr Crouch told Fairfax Media.

Ambient, which was placed into voluntary liquidation in 2014, had dozens of high-profile clients including the Victorian government, major banks, the Greens, the Victorian Aids Council and a host of leading retail brands.

The investigation touches on a major question facing Australia's advertising and media industry about whether clients are getting what they pay to receive.

This question has taken on major force with the surge of digital advertising, where eyeballs or page impressions can be hard to measure and where unethical practices by unscrupulous players can sometimes be disguised.

Fairfax Media has obtained statements from several Ambient staff also suggesting that some clients may have paid for advertising that was never actually supplied.

"On many occasions I would get feedback from my clients that they could not find their posters in the markets they had ordered. On occasion I would drive to the sites the posters should have been displayed and I could not find them," one staff member said.

The investigation is also examining financial dealings between Ambient and the Titan Media Group, which collapsed after it was backed by high-profile business figures including Australia Post deputy chairman Brendan Fleiter and BRW rich lister Patricia Ilhan, the wife of the late businessman John Ilhan.

Titan Media was founded by Mark Fishwick, who also founded Ambient Advertising.

"I'm trying to reconstruct the accounts. We are undertaking a forensic investigation," Mr Crouch said.

Early investigations have estimated that Ambient's clients may have paid for up to $1.6 million of phantom advertising, involving street posters ordered but never actually produced.

Mr Crouch is also examining the way Ambient's profits were managed by Mr Fishwick and fellow director Milan Bozic, as well as the shifting of advertising contracts from Ambient to another company prior to Ambient being placed into voluntary liquidation by Mr Fishwick.

It is understood $5.8 million from Ambient's accounts were used as management fees.

Mr Crouch declined to attribute blame to any person inside Ambient for the alleged phantom advertising or business practices under investigation, saying his inquiry was ongoing.

Mr Fishwick, who was formerly an independent director of delisted satellite company NewSat and is a well-known advertising industry figure, said he had no knowledge of any improper practices inside Ambient and would co-operate with the investigation.

When contacted by Fairfax Media, Mr Bozic said: "I have co-operated fully with the special purpose liquidator Mr Nicholas Crouch and will continue to assist him with his inquiries. I would encourage all parties to do the same."