The government is sceptical of a plan for a fast passenger rail network linking Auckland, Waikato and the Bay of Plenty.
The transport lobby group Greater Auckland has proposed a $1.45 billion rapid rail service linking Auckland to Tauranga, Rotorua, Cambridge, Hamilton and Te Kuiti.
Initially, refurbished railcars would be used, but ultimately there would be high speed tilting trains similar to those used in Switzerland.
Greater Auckland said local and central government would have to help with the cost.
In a statement, the Minister of Transport Simon Bridges said the government was always open to ideas for improving New Zealand's transport system.
But he said the Greater Auckland proposal seriously underestimated capital costs, and any new service would have to be backed by a sound business case, be commercially viable and be supported by key stakeholders.
The New Zealand Transport Agency said there were many operational matters that would need to be considered in detail to understand the implications of the proposal.
The agency said it was already investing in rail and road in the region to make transport easier.
Greater Auckland has said the costs were not large when compared with routinely-announced roading projects.
The plan has received backing from the mayors of Hamilton and Tauranga.