The National Party says a report into shifting the Ports of Auckland to Whangārei or Tauranga is a thinly disguised attempt to promote rail.
A working group is looking at ways to improve the freight supply chain in the upper North Island, and has released an interim report.
It said shifting the Ports of Auckland freight to Northport in Whangārei could bring major economic benefits to the region, but a train line would have to be built.
National's Transport spokesperson, Paul Goldsmith, said there was no evidence to suggest the investment would lead to better outcomes for exporters or consumers.
He said the results were predetermined, and would promote shifting Auckland's port operations to Northport, near Whangārei, and endorse a massive rail project from South Auckland.
"One of the interesting things of this report is that they say that strategic vision is more important than business cases when it comes to such major investments," Mr Goldsmith said.
"Well, I don't know where you end up with that. It just means you don't actually do the hard business cases, you just do what feels good and that's not a good way to base infrastructure investment."
However, Wayne Brown, the chair of the working group, dismissed Mr Goldsmith's accusation that group members were puppets for the coalition government's rail ambitions.
He said the group was made up of independent transport industry figures.
"We're looking at this from the point of view of New Zealand. The previous reports have been looked at from the point of view of Auckland but Auckland isn't New Zealand," Mr Brown said.
"It's certainly not export New Zealand; it's import New Zealand. We're looking at this from the point of view of the ability of the country to earn exports.
"There is lots of potential growth for exports from Northland so we're definitely looking at this from the point of view of improving New Zealand's export earnings."
Mr Brown said alternative solutions would be put through a full economic investigation.
Union backing for Northport move
Shifting Auckland's port operations to Northport was a no-brainer, said the Rail and Maritime Union general secretary, Wayne Butson. He said Northport was New Zealand's only natural deep water port, it had the capacity to expand its footprint easily, and exports from Northland were increasing.
"There's no way that Tauranga on its current footprint would be able to absorb all of the container traffic for the upper part of the North Island of New Zealand because a rail link relies on the Kaimai tunnel.
"The costs of going through with a new tunnel through the Kaimais would be far higher than the infrastructure investment doing up the North Auckland line."
Mr Butson said the high cost to run heavy rail through Auckland must be met anyway, because the infrastructure is essential.