The Federal Opposition is demanding to know whether a motor vehicle dealer who gave Prime Minister Kevin Rudd a free car has been given special favours by his office.

Mr Rudd was given a car, registration and road side assistance by Queensland car dealer John Grant. The freebies were declared to Parliament.

Treasury official Godwin Grech told a parliamentary hearing the Prime Minister's Office and the Treasurer's Office made representations to him on behalf of a car dealer about the Government's multi-million dollar trust set up to support dealerships.

The trust, known as OzCar, offers finance to car dealerships that have been affected by the global credit crunch.

Mr Grech would not confirm if the calls were about Mr Grant, but said they were straightforward inquiries.

"Both offices became aware of a situation facing a particular car dealer and they simply wanted - as you would expect a normal ministerial office to do - refer it to the relevant part of the bureaucracy to help the ... car dealer," he said.

Liberal Senator Eric Abetz told the Senate hearing Mr Rudd should reveal if that car dealer is John Grant.

"Was there a declaration in those professional representations that there was at least the possibility of a perception of conflict of interest?" he asked.

"I think we all know what the answer is and that the answer is 'no'."

The Opposition used Question Time to demand answers from the Prime Minister, with leader Malcolm Turnbull asking "What do you have to hide?".

But Mr Rudd who told Parliament he knew nothing about it, and accused the Opposition of enganging in "politics of smear".

Federal Treasurer Wayne Swan told Parliament Mr Grant did contact his office, but the contact was straightforward and above board.

"It's an entirely normal thing that Members of Parliament or car dealers would contact the Treasury, which is establishing and putting in place this vehicle [OzCar]," he said.

"It is the case that Mr Grant did make representations to my office and he was referred on to the SPV, just like everybody else.

"I have no idea what the outcome of that was."