Trucks are best equipped to deal with New Zealand's freight task, according to the Road Transport Forum.

The Green Party has announced a policy that would see 50 percent of the country's freight carried by ship and rail within the decade.

MORE: Greens want half of NZs freight transported by ship or rail within a decade

But chief executive Ken Shirley said the policy doesn't face the realities of New Zealand's freight task.

"The policy announced is just totally unrealistic. No jurisdiction on earth has the ratios of rail and coastal shipping that they are proposing here," he said.

Mr Shirley said trucks are simply more flexible than rail and shipping.

"You can't have a train going to the supermarkets. You can't have heavy rail going to every skid sight to harvest the forests. You can't have a train going to every farm to pick up the milk."