An MP says a threat has been made to prosecute people who have lent their cars to family or friends for weddings.

Family First MP Robert Brokenshire says several of his constituents in Adelaide have received warning letters after their cars were photographed being used for weddings.

He said a letter from the Transport Department claimed anyone running transport passenger services without accreditation can face penalties of up to four years' jail or a $15,000 fine.

The owner of a vintage car said he was shocked to get a letter threatening to prosecute him for lending it to a family friend for a wedding.

Kevin May says he had been warned of the risk of jail or a fine for lending his 1969 XW GT Falcon to a bridal party.

He is demanding the warning be withdrawn immediately, saying it is an infringement of his human rights.

"I was quite upset about it at first - I thought it was a joke - and then when I studied the letter properly, you could see it had numbers on there and it was definitely an official letter and I thought 'God, blimey charlie, I was only trying to help someone out'," he said.

"I wasn't trying to get a fine - $15,000 is a lot of money."

Mr Brokenshire said the SA Government had gone mad.

"It's one thing to be out there catching rogues ... that are impersonating accredited hire car companies but when a family friend lends the Subaru or the Ford Falcon to a friend for a wedding and they receive a demanding prosecution letter, it's absolutely outrageous," he said.

"Are the Government so tight with financial management at the moment that they're hitting innocent people with significant fines?

"I mean this is absolutely wrong. A person should be able to lend their car legitimately to a friend for a wedding without intimidation or fear of a fine or prosecution."

Transport Department manager for passenger transport, Bill Gonis, said the letters were only to inform people what was legally allowed.

"It's a letter of warning and advising them if they want to speak to someone at the office we are more than happy to help them with any inquiry they may have," he said.

"If they're simply just providing that service for a friend of theirs that's fine, that's the end of that. Again I just have to make that clear that we're not prosecuting people who are simply driving their friends around.

"This is just really about ensuring public safety if some of these vehicles are going out, [ensuring] that some of these vehicles are roadworthy."