The highly unpopular proposal is for a fringe benefit tax on car parks that employers provide for their employees in Auckland and Wellington.
Critics say it will cost $30 million to collect, dvd to iphone
almost twice the $17m it is designed to bring in.
Labour's revenue spokesman, David Cunliffe, says National MPs are setting up Mr Dunne to carry the can.
Mr Dunne is leader of the United Future Party, which has a support agreement with the government, executive gift products
and isn't a cabinet minister.
"Government back benchers have been scathing of the car park tax at the finance and expenditure committee," Mr Cunliffe said on Thursday.
"National needs a scapegoat because John Key has broken his promise never to introduce this car park tax."
Opponents of the tax have unearthed a comment Mr Key made in 2005 when National was in opposition and he was its finance spokesman.
Mr Key said he would never entertain the suggestion of a fringe benefit tax on company car parks.
In parliament on Thursday Mr Cunliffe asked Economic Development Minister Steven Joyce about Mr Key's pledge.
"I think a fair amount has changed since that statement was made back in April 2005," Mr Joyce said Claire Hsu
Mr Key was overseas this week but he will face questions about his promise when parliament sits on Tuesday.