Questions are being raised about whether legal powers should be given to the Advertising Standards Authority to enforce its decisions .

Yet another complaint - the 16th - has been upheld by the Authority against offensive branding on vehicles owned by rental company Wicked Campers.

It's another in a long series of complaints against the company which has sparked outrage and upset with its provocative branding.

Wicked campervans are known to sport depictions of pop culture characters in compromising situations. The vehicles also carry a quote - usually of a sexual nature - on the rear boot door.

The company did not reply to a request from the ASA's Complaints Board for a response.

Nick Russell a partner at the law firm Chen Palmer pointed out the ASA is a voluntary regulatory body, and unless companies sign up to it, they are not bound by its decisions.

"There's nothing that requires Wicked Campers to abide by any decisions that the authority might make and because the advertising space they use is their own vans, there's nobody else to tell them that they can't do it," Russell said.

If the Authority is repeatedly shown to be ineffective because of this sort of case, than the argument may increase for there to be stronger form of regulation.

But Russell said there doesn''t seem to be any political will to do so, and such legally-binding decisions raise freedom of speech issues.

A number of Wicked's campervan slogans have been ruled "objectionable" and "injurious to the public good" by the Chief Censor. If those banned slogans are seen in public, the company faces a $200,000 fine.