Prime Minister John Key accepted a free ride in a helicopter, decided it wasn't a good look and taxpayers ended up footing the $2000 bill, it was reported tonight.

TV3 said Mr Key was in the Waikato launching a cycleway in late 2009 when the helicopter pilot offered to take him to Auckland at no charge.

"When we got back to Auckland we later on decided there was just a risk that it could infer some sort of benefit or favour and so my office paid for it," Mr Key said.

Mr Key said he didn't know when he accepted the offer that the helicopter was owned by the Vela brothers, the fishing and racing millionaires who have been in the news in the past because of their support for New Zealand First leader Winston Peters and his use of what may have been the same helicopter.

Mr Key and others had flown from Hamilton to the Waikato in the helicopter but that was part of the cycleway launch and taxpayer money wasn't involved.

"What ended up happening was the helicopter pilot asked did I want to be dropped back to Hamilton or Auckland, and I said Auckland if that suited him," Mr Key said.

He could have returned to Hamilton and been driven to Auckland, but he said he had urgent meetings planned in Auckland which he needed to get to.

Mr Key wouldn't discuss the meetings, saying they were "security related".

Documents obtained under the Official Information Act show Mr Key needed retrospective approval to pay for the helicopter ride.

It is the only time since he became prime minister that he has used a privately-owned helicopter.

He has used Air Force helicopters, and was criticised by Labour and the Greens last month for flying in one from the V8 races in Hamilton to Auckland to attend a golf club dinner.

Opposition MPs may raise the helicopter ride in Parliament tomorrow.