National wants more answers about plans for Northland's rail network, as its profitability is called into question.
A new report reveals an upgrade will only make a return on investment if Northport's operations are expanded.
In the best-case scenario, the report gives the investment a benefit-cost-ratio of 1.19, meaning for every $1 spent there would be a $1.19 return. Without an expansion to Northport's operations, the benefit-cost-ratio is only 0.32.
National Party transport spokesperson Paul Goldsmith says discussions need to happen.
"We need to have a clear rationale on why you want to spend a billion on rail going north, as opposed to other pressures on the transport network."
The plans are expected to cost $730 million in the first four years. The Ministry of Transport estimates the total cost at $1.3b over 40 years.
Goldsmith says he needs more answers.
"I'm not saying we shouldn't be investing in rail, but there hasn't been a clear assessment of the costs and benefits that I've seen yet. We'll look at the details... with interest."
Associate Transport Minister Shane Jones has hinted to Newshub that there will be great attention on rail in the upcoming Budget.
"The reality is that if you're in the business of nation building, you cannot capture every perceived benefit over 50 to 100 years through one consultant's report," he told Newshub Nation.
Goldsmith said Jones was putting his leader Winston Peters' former electorate of Northland first, at the expense of the rest of the country.
"Nothing that we've seen from Shane Jones in terms of his spending would give any confidence he's gone about this in an open and transparent way where he's focused on the best outcome for all New Zealanders."
The majority of the benefits are expected to come from easing congestion on Auckland and Northland roads, by moving freight on rail, rather than trucks.