More than 1100 applications have been made to drive heavier or longer trucks on highways and local roads - with about half coming from the rural sector.
The high productivity motor vehicle permit scheme allows trucks to be up to 2 metres longer or to carry more than the standard 44 tonnes on approved routes.
Transport Agency figures reveal 47% of the 1123 permit applications made in the past 13 months have been approved. About one third have been denied and the remainder are still being processed.
A quarter of all applications have been made for routes in Waikato - with just 65 approved so far.
Transport Agency spokesperson Andy Knackstedt says that in about half of cases where an application has been declined, the reason is that bridges are not strong enough on the intended route, or for other roading restrictions.
Forest Owners Association transport committee chair Brian Pritchard says the number of approved applications is about where they thought it would be.
He says until many bridges are strengthened, there will be a cap on how productive operators can be.
The Government estimates improved productivity as a result of the permit regime could eventually boost GDP by as much as $500 million annually.