Transport Minister Gerry Brownlee says Auckland will have to look at selling assets to fund its share of the $2.86 billion underground rail link.
Auckland Council wants to fund its half of the project through rates rises, road charges or a combination of both, but Mayor Len Brown has ruled out selling shares in Auckland Airport and Ports of Auckland.
Mr Brownlee told TV3's The Nation that Mr Brown was entitled to his views on asset sales, but the issue would "inevitably be on the table".
"That's his position and I think he's entitled to go to the ratepayers in the coming election and say that," he said.
"But as we come up to the point where we're looking at the business case, which is about how it's funded and all the other aspects of it, he'll have to have some answers".