Campaigners are calling for plans for a vehicle tunnel under Auckland's Waitemata Harbour to be ditched in favour of a train-only option.
The plan currently being investigated features two three-lane car tunnels which will also have capacity for a rapid transit system.
But the Campaign for Better Transport's Cameron Pitches said it made no sense to prioritise cars, given it would increase traffic congestion into the city.
"We have enough capacity for cars and trucks. It is rapid transit capacity that Auckland lacks," he said.
He said a rail crossing would provide more capacity and generate less carbon and has launched a campaign - North Shore Rail - to support it.
Mr Pitches said the road option would cost $4.6 billion, while a rail tunnel would be $1.5 billion.
But according to the NZTA's 2010 estimates of the project, a full rail network out to Albany - which would have to be built along with the crossing - would cost about $11b.
A decision on the plan for ta third harbour crossing is currently scheduled for 2017. It is estimated it will take five to seven years to build.