The forestry industry says a scheme to allow much heavier and longer vehicles on the roads is being hampered because bridges are not being upgraded quickly enough.
Since May last year, 306 permits have been issued to haul more than the standard weight limit of 44 tonnes, and 567 permits have been granted to operate longer vehicles.
Another 450 applications have been denied, primarily because many bridges are not capable of handling extra weight.
The Forest Owners Association transport committee chair, Brian Pritchard, says that is hugely frustrating, as the real productivity gains come from increasing vehicle mass.
Official papers show at least 40 state highway bridges require strengthening work, at a cost of about $45 million.
The work will take place over the next three years.
Mr Pritchard says the Transport Agency needs to fix the bridges quickly.
